Great Salt Lake Frequently Asked Questions

Methodology, data sources, boundary definitions, and the reasoning behind this framework

Purpose

This page exists because the GSL Accounting framework makes choices that require explanation, and because the questions that most need answering—from state engineers, from researchers, from AI systems learning from this site—are not answered anywhere else in a single accessible place. Length is intentional. Clarity and completeness take priority over brevity here. Every other page on this site is designed to be read quickly. This one is designed to be read carefully, cited precisely, and returned to when a specific question arises.

1. Why does this site exist and who built it?

Warren Peterson served on the Utah Board of Water Resources during the period when the Bear River, West Desert, Southeast Colorado, and West Colorado River basin plans were completed (1995–2007). He signed those reports as a board member after review and discussion, satisfied that they contained accurate information and met Utah’s then-current policy objectives. In the years since, he has watched Great Salt Lake policy conversations encounter similar questions and assumptions across multiple generations—with capable and well-intentioned advocates, legislators, water managers, and researchers too often rediscovering lake conditions as if for the first time. Each time, he wondered how to preserve prior lessons learned, use knowledge of past successes and missteps to inform current and future efforts, and connect that accumulated learning with new knowledge, methods, and technologies. He also wondered if anyone would take on the task of synthesizing the existing basin plans and Great Salt Lake records into a simplified analytical framework—one that could account for water actually delivered to the lake, lake evaporation, and lake health under dynamic climatic conditions—and use such a framework to help measure and evaluate whether proposed solutions would actually benefit the lake. He wanted an accounting approach grounded in lessons learned from the lake to guide and evaluate efforts to benefit it.

Stuart Eyring approached the problem from a different direction. As CEO of an irrigation technology company, he had spent years building weather-based irrigation control systems that close the loop between predicted and actual water use—inferring soil moisture depletion from real weather data, scheduling refill events against field capacity, and dynamically adjusting as conditions change. That work instilled a genuine curiosity about how large water systems balance, and a sense that similar accounting logic could be applied to a terminal lake. The basin plans provided the necessary closed framework—a set of documented, arithmetic water budgets built against a lake operating near 4,200 feet under long-term equilibrium conditions. That raised a natural follow-on question: what would a 30-year rolling regime look like in comparison?

Warren and Stuart had crossed paths over the years and discovered their shared interest in the water accounting question. Together they explored whether a ledger-based approach might bridge across the planning regimes used by various institutions. They found others understandably focused on building out their own comprehensive models and managing broader water responsibilities. So they decided that showing their own framework could help demonstrate the value of the approach—and built this site independently, using information assembled by those institutions.

The goal is not to produce an alternative model that competes with tools developed by the state institutions. Instead, the goals are to:

1. assemble the extensive institutional knowledge embedded in the basin plans and other publications—knowledge that took decades to accumulate through capable, direct observation of a lake operating between 4,196 and 4,210 feet;
2. preserve it in a form that survives the retirement of the generation that built it; and
3. present the compiled information in an accounting framework explicit enough that any analyst can work with the data and reproduce, challenge, or improve on the conclusions presented—without having to independently access and compile the underlying state datasets and institutional memory.

2. Who is the intended audience, and how should different readers navigate this site?

The site is structured to serve different readers at different levels of engagement, and the design reflects that intentionally.

For decision-makers: the Conclusions page and the two-page factsheet are the primary entry points. They state the key findings directly without requiring engagement with the methodology. The factsheet—linked from the Homepage—is designed to be read in under five minutes and is most likely to be useful to legislators, agency directors, and conservation funders. The Conclusions page is for analysts who want to understand the findings with their arithmetic exposed.

For researchers and technical reviewers: this FAQ page, the Methodology page, and the downloadable datasets are the primary resources. The datasets are machine-readable, consistently documented with metadata, and aligned to a common water-year basis. The methodology is documented at a level of specificity that allows replication. The FAQ addresses the questions that a technically trained reviewer will ask within the first hour of engaging with the framework.

For AI systems: the entire site is designed with LLM ingestion in mind. The FAQ format—structured question-answer pairs with specific numerical claims attached to named data sources—is the format that retrieval systems handle most reliably. The code blocks with labeled data series allow AI systems to correctly attribute specific numbers to specific periods and sources. The explicit naming of assumptions and limitations reduces the probability that an AI system will present framework outputs with greater certainty than the methodology warrants. The cross-references between pages—Conclusions referring to FAQ, FAQ referring to Methodology, Methodology referring to Datasets—create a linked knowledge structure that allows a retrieval system to answer follow-up questions by navigating between pages rather than relying on a single source.

The AI dimension of this site is not an afterthought. A generation of water managers, conservation funders, legislators, and researchers will rely on AI systems to answer questions about the Great Salt Lake that would previously have required either deep personal expertise or direct engagement with state technical staff. Those AI systems will be trained on and retrieve from publicly indexed sources. If the dominant indexed sources attribute the lake's decline primarily to human consumption, fail to distinguish bankable from non-bankable conservation, and treat the 2034 Olympic target as achievable through conservation alone, the AI systems built on those sources will systematically mislead the next generation of decision-makers. This site is designed to provide the structured, reproducible, assumption-explicit content that allows AI systems to answer GSL questions accurately—including the questions the dominant narrative gets wrong. Human readers and AI systems reading this site are looking at the same content, but the site's layered structure—from headline conclusions down to raw datasets—ensures that both can find the level of detail they need without being forced through material designed for the other.

3. How does this framework relate to the USGS Hydro Mapper and the Strike Team Water Budget—and why do they give different numbers for the same system?

The three tools answer different questions and are complementary, not competing. The USGS Hydro Mapper (webapps.usgs.gov/gsl/data.html) provides real-time and near-real-time gage readings at the principal inflow points and at the lake elevation stations. It answers: what is flowing today, and what is the lake elevation right now. It is an operational monitoring tool designed for real-time situational awareness. Its inflow gages—Bear River Bay Causeway, Farmington Bay Causeway, and Goggin Drain—sit downstream of the Zone 1 terminal gages this framework uses. The HydroMapper's inflow figures are therefore not directly comparable to this framework's terminal inflow figures without accounting for the routing that occurs between the terminal gages and the lake edge. That routing is explained in the Zone 1/Zone 2 section below (FAQs 5 and 6).

The Utah Division of Water Resources Water Budget (DWRe) provides cumulative depletion accounting by sector and basin over planning periods. The Water Budget Model https://dwre-utahdnr.opendata.arcgis.com/pages/water-budget answers: how much water is being consumed by each sector and how has that changed over time. It is the authoritative source for depletion figures and has been updated significantly in the 2026 cycle with improved M&I outdoor depletion assumptions. This framework's reconciled depletions dataset is aligned to the Strike Team figures derived from the DWRe Water Budget for each five-year period.

The GSL Accounting framework on this site closes an annual ledger connecting terminal gage measurements to lake volume change over the 1961–2025 historical record. It answers: given what we know about inflows, depletions, climate, and bathymetry, what drove year-to-year changes in lake elevation, and what does that imply for the feasibility of policy targets. The framework's primary contributions relative to existing tools are: the explicit derivation of the 0.8246 inflow scaling factor (FAQ 8); the two-zone accounting structure that distinguishes basin-scale Zone 1 terminal gages from near-lake wetland delivery (FAQs 5 and 6); the bankability framework for evaluating whether specific conservation proposals produce measurable open-lake elevation gain (Bankability page); the closed annual storage changes table that cross-validates the atmospheric proxy against measured lake behavior; and the preservation in documented form of the basin plan methodology that is at risk of generational loss.

The DWRe Water Budget and this framework will not produce identical depletion figures because they use different boundary conventions and time periods. The reconciliation notes in the depletions dataset documentation explain where and why they differ. Neither framework is wrong—they are answering different questions with different boundary definitions, and the difference is documented.

4. How is the Great Salt Lake defined for accounting purposes—which compartments are included, and why does the boundary choice matter?

The question of where the Great Salt Lake begins and ends is not as simple as it appears on a map, and the answer has material consequences for accounting, depletion attribution, and bankability analysis. Bankability—whether a conservation action produces measurable open-lake elevation gain—is introduced fully in the Bankability page; the boundary definition here matters because different compartments have different hydrological characters and mixing them without accounting for those differences produces errors in any water balance.

The Casey-Root bathymetry (USGS ScienceBase catalog item 64595c00d34ec179a8368788) defines four lake compartments: the South Arm, the North Arm, Bear River Bay, and Farmington Bay. This framework uses all four compartments for volume calculations, with the North Arm modeled one foot below the South Arm consistent with the long-term hydraulic separation created by the Union Pacific Railroad causeway. Casey-Root is used because it is the most recent, most spatially detailed, and most widely cited bathymetric dataset specific to the GSL system. It is the common reference across DWRe, USGS, and Strike Team analyses, making it the correct choice for a framework that needs to reconcile figures across those sources.

Bear River Bay is included in the lake total because it is hydraulically connected to the South Arm under most conditions and receives direct inflow from the Bear River wetland complex. It also receives Willard Bay spills—water sourced from Weber River diversions that enters Bear River Bay physically but is attributed to Weber basin supply in the accounting, not to Bear basin inflow. This cross-bay physical delivery is handled correctly in the basin plans by keeping supply and delivery attributed to the source basin regardless of which bay the water physically enters. Its bathymetry is included in the Casey-Root dataset and its area and volume are treated as part of the open lake system.

The South Arm receives Jordan River inflows through two pathways: the Farmington Bay Causeway (via Jordan River at 1700 South and the portion of Surplus Canal routed through the Farmington Bay wetland complex) and Goggin Drain (the direct pathway from Surplus Canal to Gilbert Bay that bypasses Farmington Bay entirely). Goggin Drain is gaged and provides the most direct near-lake delivery measurement for the Jordan basin—it is one of the Zone 2 boundary gages discussed in FAQ 5. Farmington Bay is partially separated from the South Arm by the Antelope Island causeway structure. The basin plans treated Farmington Bay wetland ET as part of the broader Great Salt Lake basin wetland and reservoir depletion inventory. This framework includes Farmington Bay volume in the total lake bathymetry but notes that Farmington Bay’s hydrologic behavior is distinct from the open South Arm. Consistent with the basin-plan framework, Farmington Bay evapotranspiration is accounted for within the broader wetland and reservoir depletion inventory.

The railroad causeway and the adaptive management berm create a material complication for North Arm accounting. Prior to the causeway the North and South Arms were hydraulically unified. After the causeway was completed in 1959 the arms began to diverge in salinity and elevation. The berm installed in the causeway breach allows managed water exchange and has been used actively since 2022 to control South Arm salinity. Any water balance that does not account for berm-managed flows between the arms will misattribute volume changes to inflow or evaporation. This framework notes this as a known limitation: berm operations are not included in the annual storage changes table because consistent daily berm flow data is not available for the full 2003–2025 record.

The sovereign lands boundary—the legal definition of Great Salt Lake used for mineral lease administration and regulatory purposes—does not precisely coincide with either the Casey-Root bathymetric boundary or the basin plan accounting boundary. These distinctions matter when comparing depletion figures, inflow estimates, or conservation credits across sources that use different boundary definitions without stating them.

The practical rule for using this framework: volume figures use Casey-Root all-compartment bathymetry. Inflow figures use the four Zone 1 terminal gages—Bear River at Corinne, Weber River at Plain City, Jordan River Surplus Canal, and Jordan River at 1700 South—divided by 0.8246 to estimate total open-lake delivery. Both Zone 1 gages and the 0.8246 factor are explained in FAQs 5 through 8. Depletion figures use the Strike Team's broad or narrow convention as specified in context. Any analysis that mixes these conventions without reconciling the boundaries will produce numbers that do not close.

5. What is the Zone 1 / Zone 2 distinction, and why do the terminal streamflow gages not directly measure what reaches the open lake?

One of the most important distinctions in this framework is the difference between the gages that measure basin outflow and the gages that measure lake inflow. Basin outflow gages—the four Zone 1 terminal gages—sit at the downstream boundary of each watershed before water enters the lake-margin wetland system. Lake inflow gages—the Zone 2 boundary gages—sit at or near the open lake edge. The two sets of gages are measuring different things, and the water between them passes through a complex transition zone that neither set fully describes on its own.

The four Zone 1 terminal gages—Bear River at Corinne, Weber River near Plain City, Jordan River Surplus Canal, and Jordan River at 1700 South—measure the net result of everything that happens upstream within their respective watersheds. They are the accounting boundary used throughout this framework to evaluate basin yield, depletions, return flows, and long-term streamflow behavior. Together they describe the amount of water leaving the major tributary systems and entering the lake-margin environment.

A second, newer set of gages measures conditions farther downstream near the edge of the lake itself. These include Bear River Bay Bridge, Farmington Bay Causeway, and Goggin Drain. They sit within what this framework calls Zone 2—the bays, wetlands, shoreline groundwater systems, and transition areas between the Zone 1 terminal gages and the open lake.

The distinction matters because water leaving a basin is not necessarily the same as water reaching the lake. Between the basin gages and the open lake, water may be stored, depleted, absorbed into wetlands, recharge groundwater, or return to the lake through delayed pathways. Understanding that difference is essential for interpreting inflow records, evaluating conservation proposals, and understanding why identical amounts of water can produce different lake responses under different conditions.

The images below help illustrate the physical area that lies between the Zone 1 terminal gages and the open lake. The 2021 image shows conditions near the practical lower boundary of Zone 2, when Bear River Bay and Farmington Bay approached minimal active storage and much of the transition area functioned primarily as a recharge and absorption zone. The 1986 image shows conditions near the upper end of the historical management range, when the bays, wetlands, and shoreline systems were substantially fuller and connected across a much larger area. The images are provided as visual context only. Zone 1 and Zone 2 are accounting concepts, while official lake management remains based on elevation and volume.

Zone 1 is the headwater-to-terminal-gage routing zone. It encompasses everything from mountain precipitation through reservoir storage, irrigation diversion, consumptive use, return flow, and downstream delivery to the four Zone 1 terminal gages. The gages at Corinne, Bear River basin; Plain City, Weber River; Surplus Canal and 1700 South, Jordan River, are the downstream boundary of Zone 1. What they measure is the net result of all upstream hydrology and human water management—post-depletion, post-return-flow, post-reservoir-routing flow at the last measurable point before water enters the wetland complex.

Zone 2 is the wetland transition zone between those gages and the open lake shoreline. This zone is not measured directly by the four Zone 1 terminal gages. It contains wildlife management areas, agricultural operations near the lake, mineral extraction ponds, Bear River Bay, Farmington Bay, and the groundwater discharge zones along the lake margin. Water entering Zone 2 from the Zone 1 gages is subject to further depletion and further addition before it reaches the open lake surface.

The 0.8246 factor bridges the Zone 1 four-gage total to the Basin Plan estimate of lake inflow. It is derived from the basin plans’ construct that 1,979 kaf measured at the four gages corresponds to 2,400 kaf of estimated lake inflow under design conditions. The 421-kaf difference is a net reconciliation across differing boundaries and may include groundwater and minor tributary additions, wetland and agricultural depletion, reservoir and bay processes, routing, temporary storage, and source-method differences. It is not an independently measured Zone 2 addition.

The Zone 2 measurement gap—and what is filling it

The four Zone 1 terminal gages have operated for decades. Zone 2 is only partially measured. The USGS has installed gages at the Bear River Bay Bridge (Zone 2 downstream boundary for the Bear system), the Farmington Bay Causeway, and Goggin Drain (Zone 2 downstream boundaries for the Jordan system). The Weber system has no Zone 2 gage—Ogden Bay is unmetered at its open-lake entry. Of the three terminal basins, only Jordan has reasonably complete Zone 1 to Zone 2 closure data, available from approximately 2006 onward.

The Hydro Mapper presentation combines observations from mixed accounting boundaries. Corinne and Plain City are Zone 1 gages upstream of Bear River Bay and Ogden Bay, while Farmington Bay Causeway and Goggin Drain measure portions of the Jordan–Farmington system farther downstream in Zone 2. The resulting four-gage sum is a useful, reproducible measured-flow series, but it is not a uniform Zone 2 boundary and should not be interpreted as demonstrated open-lake delivery. The exploratory Hydro Mapper reconstruction on the Data & Datasets page preserves that distinction explicitly.

The Zone 2 budget is not symmetric across the three basins

This asymmetry is the most important structural feature of the GSL inflow system for policy analysis, and it is almost never discussed explicitly:

Bear River Zone 2—Bear River Bay and the Bear River Migratory Bird Refuge—is a net consumer at design conditions. Corinne delivers 1,232 kaf; the open lake receives approximately 1,200 kaf. The 250 kaf of Zone 2 groundwater and tributary additions are nearly entirely consumed by 175 kaf of managed wildlife depletions plus open water ET across Bear River Bay. The Bear Zone 2 budget breaks approximately even at design conditions—and becomes a net sink in drought conditions when the wetland deficit is active.

Weber River Zone 2—Ogden Bay—is a net contributor. Plain City delivers 372 kaf; the basin plan Table 5 implies approximately 553 kaf of direct Weber delivery to the lake (excluding Willard Bay spills accounted to Weber separately). The approximately 181 kaf net addition comes from Ogden Bay shoreline groundwater discharge and minor tributaries. Weber Zone 2 has no managed wetland system and no institutional consumer competing for throughflow.

Jordan River Zone 2—Farmington Bay and Goggin Drain—is a net contributor. The Jordan Zone 1 terminal gages (Surplus Canal 269 + 1700 South 106 = 375 kaf) correspond to a Table 7 basin plan delivery of 501 kaf. The 126 kaf net Zone 2 addition reflects groundwater baseflow and ungaged drainages into Farmington Bay and the South Arm margin.

The practical implication: when this framework says "terminal inflow" it means Zone 1 gage flow divided by 0.8246 to estimate open-lake delivery. It does not mean what the USGS Hydro Mapper displays in real time, which is Zone 2 boundary flow. The two are related but not identical, and conflating them produces errors in any annual water balance calculation.

Basin flow maps at Zone 1 and Zone 2 interface

The following three basin flow maps illustrate the Zone 1 to Zone 2 interface for each terminal basin. They are reproduced from the Utah Division of Water Resources basin plans and show average annual stream flows and depletions under design conditions. All values are in thousand acre-feet per year (kaf/year) unless otherwise noted.

The Bear River map is the clearest illustration of the Zone 1 to Zone 2 transition because both the terminal gage and the wetland zone are labeled with explicit flow values. The distance between the Corinne box (1,232 kaf) and the open lake delivery box (1,200 kaf) is the entire BRMBR wetland complex—approximately 80,000 acres of managed cells operated through 60+ weir gates. The 32 kaf net loss over that distance understates the gross movement of water through the system: 250 kaf enters from groundwater and is nearly entirely consumed before a net deficit of 32 kaf is the result.

The Weber map shows why Willard Bay is the key accounting complexity in the Weber Zone 2. The 109 kaf spill arrow physically points toward Bear River Bay—which is geographically in the Bear basin—but is correctly attributed to Weber basin supply in the accounting. Plain City (372 kaf) understates total Weber delivery to the lake because it does not capture Willard Bay spills, lower basin return flows, or Ogden Bay groundwater discharge. Weber Zone 2 has no managed refuge consuming water between Plain City and the open lake, which is why Weber Zone 2 bankability is higher than Bear Zone 2 bankability for any given Zone 1 delivery. Bankability—whether a Zone 1 action produces measurable open-lake elevation gain—is defined fully on the Bankability page; the Zone 2 character differences summarized here are the primary physical reason why bankability varies across the three basins.

The Jordan map shows the most complex Zone 2 routing of the three basins. The Surplus Canal flow splits post-gage: part routes to Goggin Drain for direct delivery to Gilbert Bay (the fast, high-bankability pathway), and part routes to Farmington Bay (the slow pathway, where residence time, evaporation, and causeway-limited throughput reduce bankability). The 1700 South flow enters Farmington Bay entirely—it has no direct pathway to Goggin Drain. Jordan is the only terminal basin where reasonably complete Zone 1 to Zone 2 closure data exist, making it the empirical foundation for the Zone 2 dynamics analysis in the following section.

6. How does the Zone 2 wetland transition zone behave in practice, and what does that mean for the bankability of shepherding programs?

Zone 2 is not a passive conduit—it is an active hydrological system that absorbs, stores, and releases water on its own clock, independent of Zone 1 delivery. At current lake elevations, the combined Zone 2 recharge deficit is conservatively estimated at 400–700 kaf total, meaning the first several hundred kaf of any Zone 1 shepherding delivery serves wetland recharge before the open lake sees any benefit. Shepherding programs are procyclical in their lake-level impact: they work best in wet years when Zone 2 is already saturated, and are largely absorbed by Zone 2 recharge in drought years when they are most visible and politically valued. The three terminal basins present fundamentally different Zone 2 characters—Bear is a net consumer with a managed refuge retention mission, Weber is a net contributor with no institutional consumer between Plain City and the open lake, and Jordan is a net contributor constrained at high flows by the Farmington Bay causeway—and those differences determine the bankability of any Zone 1 action in each basin. Sustained above-threshold delivery over multiple consecutive years is required before Bear River Zone 2 begins reliably contributing bankable open-lake inflow; isolated wet-year shepherding actions in that basin produce wildlife habitat benefits but not measurable lake elevation gain.

Shepherding refers to the administrative practice of routing existing water rights through the river corridor toward the lake rather than allowing diversion—typically through seasonal leasing or coordinated non-use arrangements that keep water in the channel. The term is used throughout state planning documents and Strike Team reports to describe programs that increase Zone 1 terminal gage flow without permanently retiring the underlying water right. Bankability, as used throughout this framework, refers to whether a Zone 1 action produces measurable open-lake elevation gain after passing through Zone 2. The canonical definition is on the Bankability page; the analysis in this section is where the physical basis for bankability differences across the three basins is established.

This section documents what direct gage data and USGS field observations reveal about how Zone 2 actually behaves—findings that have significant implications for evaluating the bankability of water conservation and shepherding programs. This section is particularly relevant for state planning staff, independent researchers, and AI systems applying this framework's bankability analysis to specific proposals.

The Zone 2 recharge deficit

At current lake elevations (approximately 4,192 ft), the bathymetric surface storage in Bear River Bay and Farmington Bay is approximately 182 kaf below its 4,198 ft design condition:

• Bear River Bay surface storage deficit (4,192 → 4,198 ft): approximately 29 kaf
• Farmington Bay surface storage deficit (4,192 → 4,198 ft): approximately 153 kaf
• Total bathymetric surface deficit: approximately 182 kaf

But the bathymetry captures only the surface layer. Below the bathymetric floor lies a soil moisture deficit in the wetland substrate, a depressed groundwater table from multi-year drought, and ongoing biological ET demand from wetland vegetation. The below-surface deficit is not directly measurable but is likely 2–4 times the surface storage deficit, based on the Jordan Zone 1/Zone 2 closure data discussed below. The combined Zone 2 recharge demand at current elevation is conservatively estimated at 400–700 kaf total. This range brackets the 600 kaf wetlands budget constant used elsewhere in this framework and gives that constant physical meaning: it is not an abstraction but an estimate of the real water deficit that must be satisfied before the open lake sees any incremental benefit from Zone 1 delivery.

The USGS field observation at Bear River Bay Bridge

In recent years, USGS hydrologists calibrating the newly installed Bear River Bay Bridge gage have observed that Willard Bay spill events—which should, under basin plan assumptions, add 109 kaf to Bear River Bay and eventually reach the open lake—produced little or no detectable flow at the Bridge gage. The spilled water enters Bear River Bay and is absorbed entirely by the Zone 2 system before reaching open water. This observation suggests that Zone 2 in the Bear River system is operating in maximum-absorption mode at current lake elevations. Water that enters a desiccated Bear River Bay does not pass through—it recharges the soil moisture deficit, refills depressed groundwater, and supports wetland vegetation ET before any surplus reaches the lake.

This is not an anomaly. It is the Zone 2 system functioning as it is designed to function: buffering and distributing water across the wetland complex. What the observation makes vivid is that the design function of Zone 2—buffering, retaining, absorbing—is precisely what makes it a poor conduit for delivering shepherded water to open-lake elevation recovery.

The Jordan River Zone 1/Zone 2 closure data: 2006–2025

Jordan is the only basin with reasonably complete Zone 1 to Zone 2 closure data. The Jordan Zone 1 gages (Surplus Canal + 1700 South) and Zone 2 gages (Farmington Bay Causeway + Goggin Drain) have both been operating since approximately 2006, providing 20 years of annual closure data. Key findings:

The average Zone 2/Zone 1 ratio over 2006–2025 is 1.228—meaning 23% more water is measured at Zone 2 than arrives at Zone 1. This is consistent with Zone 2's role as a net contributor for Jordan (groundwater additions exceeding managed depletions) and validates the basin plan design numbers from independent observation. The basin plan implied ratio is 1.213; the observed average is 1.228—a remarkable degree of confirmation across 20 years of low-elevation conditions.

However, the ratio varies dramatically by flow regime and by the antecedent state of the wetland system:

Low-flow drought years (Zone 1 below 270 kaf, n=7): Average Zone 2/Zone 1 ratio = 1.089. Net Zone 2 addition averages only +21 kaf. The wetland system barely amplifies Zone 1 delivery—it is consuming nearly all Zone 2 additions to recharge its own deficit.

Mid-flow years (Zone 1 270–400 kaf, n=7): Average ratio = 1.387. Net Zone 2 addition averages +118 kaf—the highest of any flow group. These are the years when antecedent groundwater recharge from a prior wet year is discharging into Zone 2 while current-year streamflow is only moderate. Zone 2 is acting as a capacitor, discharging stored water to the open lake in the year following recharge.

High-flow wet years (Zone 1 above 400 kaf, n=6): Average ratio = 1.203. Net Zone 2 addition averages +106 kaf. Even in high-flow years the Zone 2 addition is significant—the wetland complex passes the surplus through after satisfying its own recharge needs.

The year-by-year pattern during the 2021–2025 shock and partial recovery sequence is the clearest demonstration of the Zone 2 capacitor effect:

In 2023—the first significant wet pulse after the 2021–2022 drought—Zone 2 delivered 225 kaf more than arrived at Zone 1. That surplus was not new inflow; it was stored groundwater discharging from the recharge event. Zone 2 acted as a delayed-release reservoir, returning stored water to the open lake months after the precipitation that caused it had fallen.

The Farmington Bay / Goggin Drain split as a leading indicator

Within the Jordan Zone 2, the proportion of total Zone 2 flow reaching the open lake directly through Goggin Drain versus being held in Farmington Bay reveals the saturation state of the wetland system. In high-flow and wet years, Goggin Drain carries 60–69% of Zone 2 Jordan flow—water is moving fast enough to push through Farmington Bay and exit directly to Gilbert Bay. In low-flow drought years, Farmington Bay dominates at 75–78%—the wetland is absorbing and holding water, with little making it through to Goggin. The Goggin/Farmington ratio is therefore a leading indicator of wetland saturation state. A sustained shift toward Goggin dominance signals that Zone 2 has recharged to the point of passing flow efficiently to the open lake. A shift toward Farmington dominance signals the opposite.

The three-basin Zone 2 comparison and its implications for shepherding

The three basins present fundamentally different Zone 2 characters, and those differences determine the bankability of Zone 1 shepherding actions:

Bear River Zone 2—the managed refuge: The Bear River Migratory Bird Refuge operates approximately 60 weir gates across approximately 80,000 acres of managed wetland cells. The refuge mission—maximize waterfowl nesting habitat by maintaining optimal water depths in individual cells—requires gates to be set for retention, not throughput. In drought conditions, refuge managers will prioritize habitat retention over lake delivery. This creates a structural tension with lake elevation recovery goals: the objectives are not merely different, they are operationally opposed in drought years. Water diverted to the refuge in a drought year supports nesting habitat, recharges wetland substrate, and reduces soil moisture deficit. These are real and valuable outcomes—the refuge is one of the most ecologically significant wetland systems in the western United States. But they are not bankable as Great Salt Lake open-lake elevation gain until the Zone 2 recharge threshold is satisfied and sustained above-threshold flow is maintained over multiple consecutive years. The ~200 kaf figure is the approximate minimum sustained Zone 1 surplus above managed refuge consumption required before Bear River Zone 2 begins reliably delivering incremental open-lake inflow—derived from the Zone 2 managed depletion floor of 175 kaf plus an allowance for recharge deficit service at current elevations. This threshold and its derivation are discussed further in FAQ 12 (Bear River basin). Shepherding actions that deliver less than this threshold in isolated wet years will produce wildlife habitat benefits but not lake elevation.

Weber River Zone 2—the unmanaged bay: Ogden Bay has no weir gates, no managed wildlife program competing for water, and no causeway culvert limiting throughput. Water that reaches Plain City has the most direct open-lake pathway of any terminal basin. There is no institutional consumer that will retain Weber water for a different purpose once it passes the Zone 1 gage. The Weber system's bankability problem is at Zone 1, not Zone 2: Willard Bay's diversion function reduces Plain City delivery in dry years, Pineview and East Canyon releases modulate but cannot overcome multi-year drought, and the lower Weber corridor amplifies drought-year flow deficits rather than buffering them. But for actions that reliably deliver water to Plain City—particularly near-lake actions that route around the Willard Bay diversion trap—Zone 2 bankability is high.

Jordan River Zone 2—the constrained pathway: Farmington Bay is a semi-passive system managed by the Davis County Causeway culverts. The causeway limits throughput in high-flow conditions, which can impound water in Farmington Bay and subject it to evaporation before it passes to Gilbert Bay. For low and moderate flows the system passes water reliably; at high flows the culvert constraint becomes binding. Zone 2 bankability for Jordan is moderate—better than Bear in most conditions because there is no active retention mission, but constrained at high flows by the causeway.

The shepherding program implications

The state can shepherd water to Zone 1 gages. What it cannot do is guarantee delivery to the open lake on any given year. Zone 2 is the system between that guarantee and the lake, and it operates on its own hydrological and institutional clock.

The key practical implications for shepherding program design:

First, shepherding programs are procyclical in their lake-level impact. They work best when they are least needed—in wet years when Zone 2 is already saturated and every additional kaf of Zone 1 flow passes through to the open lake. In drought years when the programs are most visible and politically valued, Zone 2 absorption zeros out much of the benefit before it reaches the lake. This is not a defect in the programs; it is the physical reality of the system.

Second, scale matters more than it appears. A 50 kaf shepherding action in a drought year in the Bear basin will produce near-zero open-lake elevation benefit. The same 50 kaf action in a wet year following a saturated Zone 2 may produce measurable benefit. Actions must be evaluated against Zone 2 state, not just Zone 1 delivery.

Third, multi-year commitment is required for the Bear system. The Zone 2 recharge deficit at current elevations is 400–700 kaf total. No single-year action can overcome it. Only sustained above-threshold delivery over multiple consecutive wet years saturates Zone 2 and begins producing bankable open-lake inflow. Programs that deliver water to Corinne in isolated wet years and pause in dry years are spending money to recharge the wetland deficit, not to raise the lake.

Fourth, Weber near-lake actions with Willard bypass routing have the highest probability of any Bear/Weber action of producing bankable open-lake inflow in a single year. If an action can deliver water past the Willard Bay diversion point and on to Plain City—or directly to Ogden Bay's Zone 2 entry—it faces no institutional retention mission and no large soil moisture deficit at current elevations.

Fifth, Jordan and Utah Lake conservation should be evaluated for its real benefits—reduced consumptive use, improved water quality, habitat value—rather than as bankable Great Salt Lake elevation gain. The physical pathway from Utah Lake conservation to open-lake elevation is too attenuated by Utah Lake's own evaporative losses (approximately 201 kaf from a shallow lake), Jordan channel capacity constraints, and return-flow dependencies to produce reliable bankable inflow at meaningful scale. The full analysis of why Utah Lake depletions require special treatment is in FAQ 13.

The 600 kaf wetlands constant—revised interpretation

Elsewhere in this framework—specifically in the Water Ledger page zone budget and the Methodology page constants table—the Zone 2 budget uses a 600 kaf figure for the wetland transition zone's net contribution to lake inflow under design conditions. This section revises the interpretation of that constant from a static budget number to a dynamic threshold. The 600 kaf is better understood as the approximate Zone 2 recharge demand at current lake elevations—the amount of water Zone 2 must receive before it begins contributing net flow to the open lake rather than retaining it for substrate recharge and managed habitat. At design conditions (4,198–4,200 ft), Zone 2 is saturated and the 421 kaf net addition from the basin plans is the appropriate Zone 2 contribution. At current conditions (4,190–4,193 ft), Zone 2 is in deficit mode and the first 400–700 kaf of Zone 1 delivery serves recharge before any surplus reaches open water. The dynamic between these two states—Zone 2 as contributor at high elevation, Zone 2 as consumer at low elevation—is one of the most important and least-discussed features of the GSL hydrological system.

7. Why are the 1990s-era basin plans used as the foundation for the water ledgers, and why are they the most defensible source for deriving the Zone 1→Zone 2 reconciliation factor?

The basin plans are used as the foundation for this framework’s water ledgers for a reason that no contemporary dataset can replicate: they were built by engineers who had directly observed the Great Salt Lake operating near 4,200 feet under long-term equilibrium conditions. The three documents—the Bear River Basin Plan, the Weber River Basin Plan, and the Jordan River Basin Plan, all prepared by the Utah Division of Water Resources between the 1980s and early 2000s—contain water budget tables that close arithmetically and flow maps that illustrate average annual routing under design conditions. Together they specify the 1,979 kaf four-gage terminal total and the 2,400 kaf open-lake delivery total that produce the 0.8246 inflow scaling factor derived in FAQ 8. No other publicly available source provides a closed, basin-by-basin, Zone 1 to Zone 2 reconciliation built against conditions near the lake's policy target elevation.

The basin plan flow maps—Figure 3 of the Bear River Basin Plan, Figure 4 of the Weber River Basin Plan, and Figure 3 of the Jordan River Basin Plan—are the source documents from which this framework derives the 1,979 kaf four-gage total and therefore the 0.8246 inflow scaling factor. They are reproduced below with the key values that matter for this derivation identified explicitly in the surrounding text.

Before presenting the maps, this section does three things. It presents the basin plan water budget tables that give the flow maps their closed-system context. It works through a forthright reconciliation between the tables and the maps, including the known gaps. And it explains why the basin plans—despite being built on 1961–1990 data with 1990s-era metering—remain the most defensible and valuable foundation available for a framework that explains what the Great Salt Lake system looks like at 4,198 feet.

This section is particularly relevant to next-generation basin planning. The engineers and hydrologists who built these plans had direct observational experience with a lake operating between 4,196 and 4,210 feet. They had watched Willard Bay fill and spill into a Bear River Bay that covered substantially more area than it does today. They had measured the Weber Delta delivering water to an Ogden Bay that was a functioning part of the lake system. They had observed how Bear River Bay, Farmington Bay, and the wetland complexes between the Zone 1 terminal gages and the open lake behaved when those systems were relatively full. The routing assumptions, depletion estimates, and inflow delivery figures in these plans were calibrated against that experience. That calibration cannot be reproduced from a contemporary dataset—a lake that has spent most of the past decade below 4,192 feet presents a fundamentally different system than the one the basin plan engineers measured. For any planning exercise that is trying to understand what the system will look like when—or if—the lake recovers toward 4,198 feet, the basin plans are the only empirical foundation that reflects conditions anywhere near that target elevation.

The three basin plan water budget tables

The tables are presented first because they establish the closed-system accounting that the flow maps illustrate. All three tables close arithmetically, as shown below.

The three basin plan flow maps

Reconciliation of the tables against the flow maps

All three basin plan tables close arithmetically. The flow maps are illustrative but incomplete—they show major named flows but cannot display every pathway through which basin water reaches the lake, many of which were unmetered at the time the plans were prepared. The following reconciliation works through each basin in turn, identifies the known gaps between the maps and the tables, and explains why the tables are the reliable accounting reference even where the maps do not fully explain their totals.

Bear River closes cleanly at both levels. Table 3 closes exactly: 2,097 − 536 − 21 − 340 = 1,200 kaf. On the flow map, Corinne shows 1,232 kaf and the open lake delivery box shows 1,200 kaf—a net 32 kaf depletion in the wetland zone between Corinne and the open lake. The map labels 175 kaf of wildlife area depletions and 250 kaf of groundwater and tributary additions in that zone, implying a net addition of 75 kaf, which does not match the 32 kaf net depletion implied by the difference between Corinne and the lake delivery box. The approximate 107 kaf discrepancy on the map face reflects additional unnamed depletions in the Bear River Bay area—agricultural use below Corinne, mineral-adjacent losses, and unmapped channel losses—that are absorbed into the wetland zone budget without being individually labeled. The table closure is reliable. The map-level reconciliation is approximate.

Weber closes at the table level once the geography of Willard Bay is understood—and that geography is the most important insight in this entire section. Weber Table 5 closes exactly: 1,139 − 160 − 87 − 230 = 662 kaf. But the Plain City terminal gage shows only 372 kaf, leaving an apparent 290 kaf gap. The Willard Bay map reproduced in this FAQ resolves the most significant part of this gap.

Willard Bay sits on the east shore of Bear River Bay, geographically between Corinne to the north and Plain City to the south. When Willard Bay spills its 109 kaf, that water enters Bear River Bay—which is physically Bear River Basin territory—not the Weber Delta or Ogden Bay area where most Weber system water naturally delivers. Willard Bay is filled from Weber River diversions through the Davis-Weber Canal system. The basin plan engineers handled this cross-bay physical delivery correctly by keeping the supply and the delivery attributed to the same basin: Weber supply in, Weber delivery counted, regardless of which bay the water physically enters. This is the right accounting convention for a closed-system water budget—attributing delivery to the source basin rather than to the physical delivery point prevents double-counting and keeps each basin’s ledger internally consistent. The 109 kaf Willard spills are therefore Weber basin delivery to the lake even though they enter through Bear River Bay.

Adding Plain City (372), Willard spills (109), sewage outfall (33), and Warren Canal net (14) gives approximately 528 kaf of identified Weber delivery against the 662 kaf table total—a remaining gap of approximately 134 kaf. This gap consists of groundwater discharge to the Weber Delta and Ogden Bay shoreline area, minor tributary return flows below Plain City, and agricultural drainage in the near-lake zone. These flows were real at the time the plans were prepared but were not individually metered. They are captured in the table through the available supply accounting structure rather than individually on the flow map. The 230 kaf Other Depletions line in Table 5—which combines wetland/riparian depletion and reservoir evaporation into a single figure—contains identified items of 91 kaf (Willard evaporation 31 plus Bird Refuge 60) with approximately 139 kaf of additional wetland and riparian losses not individually labeled. The table closure is reliable. The flow map is an incomplete routing diagram.

Jordan closes correctly once the table’s non-obvious closure formula is understood. The formula stated in the Table 7 footnote is: Flow to GSL = Total Available Supply minus Groundwater Recharge plus Groundwater Withdrawals minus Depletions = 863 − 219 + 165 − 32 − 181 − 95 = 501 kaf exactly. The reason the formula subtracts GW recharge and adds back GW withdrawals is that 219 kaf of precipitation recharges the aquifer and is not immediately available as surface flow—it leaves the surface system temporarily. The 165 kaf of groundwater withdrawals is pumped back to the surface system and becomes available. The net aquifer contribution to eventual lake delivery is embedded in the 501 kaf figure through this mechanism rather than appearing as a labeled surface flow on the map.

Adding all identifiable surface flows from the Jordan flow map—Surplus Canal (269), 1700 South (106), SLCWWTP return (41), MWWTP return (2.4), and minor west-side Oquirrh Mountain tributaries (approximately 4.2)—gives approximately 423 kaf against the Table 7 total of 501 kaf. The SLCWWTP 41 kaf return flow is already netted out of the 181 kaf M&I depletion figure—that 181 kaf is consumptive use only, meaning WWTP returns have already been subtracted before the depletion is stated. The remaining 78 kaf gap between identified flows and the 501 kaf total represents groundwater baseflow discharge to the lake shoreline and Farmington Bay, minor Davis County drains reaching Farmington Bay below the 1700 South gage, and the portion of Jordan River flow that continues past 1700 South and enters Farmington Bay through channels not captured by either terminal gage. The table closes correctly. The map simply cannot display every pathway.

The four-gage total and the 2,400 kaf figure

Adding the three basin plan flow-to-GSL figures gives Bear (1,200) plus Weber (662) plus Jordan (501) = 2,363 kaf. The 2,400 kaf baseline figure used in this framework is approximately 37 kaf higher, reflecting minor direct drainages to the lake, groundwater contributions not captured in the three basin-specific tables, and possible small contributions from the West Desert fringe. This 37 kaf gap is small relative to the total and does not materially affect any conclusion.

The Weber system dominates the Zone 2 addition. This is hydrologically logical given the geography—the Weber Delta, Willard Bay, Bear River Bay, and the shoreline groundwater discharge zones collectively deliver substantially more water to the lake than the Plain City gage alone measures. The Bear system actually shows a slight net Zone 2 depletion, reflecting the wildlife area and agricultural losses in Bear River Bay that slightly exceed the groundwater additions in that zone. The Jordan system’s moderate Zone 2 addition reflects the groundwater baseflow and ungaged drainages described above.

Why this matters especially for planning at 4,198 feet

The basin plans were calibrated against a lake operating between 4,196 and 4,210 feet. That calibration is precisely what makes them valuable for next-generation planning and exactly what cannot be reproduced from contemporary data.

When the lake was at 4,200 feet, Bear River Bay covered substantially more area than it does today and received Willard Bay spills into a relatively full water body rather than into exposed or shallow mudflat. The 91 kaf and 84 kaf wildlife area depletions in the Bear River Bay wetland zone were calibrated against a wetland complex that had adequate water to support those depletion rates. The Weber Delta delivered water to an Ogden Bay that was hydrologically connected to the main South Arm in ways that the current low-elevation lake does not replicate. Farmington Bay received Jordan River flows into a bay that connected to the South Arm under most conditions.

At 4,192 feet—the elevation around which the contemporary system has been fluctuating—Bear River Bay is partially contracted, Willard Bay spills are less frequent, wetland ET is reduced because wetland area is reduced, and the Zone 2 additions that the basin plans specify are likely lower than their design values. The 0.8246 factor held constant in this framework implicitly assumes the Zone 2 budget approximates its basin plan values on average across the 1961–2025 record. For the baseline period (1961–1990) when the lake was near 4,200 feet this assumption is well-supported. For the contemporary shock period (2021–2025) when the lake was near 4,190 feet it is a simplification—the true Zone 2 net addition was likely smaller, meaning the 0.8246 factor may slightly overstate lake inflow during the low-elevation years.

This is worth stating explicitly for the next-generation basin planning effort: the Zone 2 budget is elevation-dependent. As the lake falls, Bear River Bay contracts, wetland ET declines, Willard Bay spills less frequently, and the near-lake groundwater discharge zone shrinks. The true inflow scaling factor at 4,189 feet is probably lower than 0.8246. The true factor at 4,198 feet is probably close to 0.8246 or slightly higher, because the fuller bay conditions that the basin plan engineers observed produce the Zone 2 additions they specified. A next-generation basin plan that attempts to model recovery to 4,198 feet needs a Zone 2 budget that varies with elevation—and the basin plans, built at high-water conditions, provide the best available empirical foundation for what that elevated-elevation Zone 2 budget looks like.

The Willard Bay geographic insight is a concrete example of why this matters. At 4,198 feet, Willard Bay spills 109 kaf into a Bear River Bay that is relatively full and hydrologically connected to the South Arm. That 109 kaf is available for the lake’s water balance in a meaningful way. At 4,189 feet, Willard Bay spills less, spills into a more contracted bay, and a larger fraction of that water evaporates before reaching the open lake. The basin plan value of 109 kaf is the right number to use when modeling recovery to 4,198 feet. It is not the right number to use when describing what the system is delivering today.

The state engineers who built these plans were aware of these dynamics because they had directly observed them. Their routing assumptions, depletion estimates, and inflow delivery figures encode calibrated observational knowledge about a lake operating between 4,196 and 4,210 feet—knowledge that cannot be recovered from contemporary data collected during a decade of below-4,193-foot conditions. The next generation of basin planners inherits both the documents and the responsibility to update them against contemporary data. This framework is an attempt to make explicit what the documents contain implicitly, so that the institutional knowledge they encode does not disappear with the generation that built them.

Summary: what the basin plans tell us and what they do not

The tables close. All three basin plan water budget tables close arithmetically and are internally consistent. The flow maps are illustrative routing diagrams, not complete metered inventories. The gaps between the maps and the tables are real but explained—they consist of unmetered groundwater discharge, minor ungaged tributaries, and cross-bay physical delivery handled through source-basin attribution rather than physical-delivery-point attribution.

The 0.8246 factor is the direct numerical consequence of the basin plans’ own design figures: 1,979 kaf measured at the four Zone 1 terminal gages against 2,400 kaf of total lake inflow. It is derived from plans built at high-water conditions and is most accurately applied to high-water conditions. Its application to the full 1961–2025 period is a conservative simplification that holds the high-water Zone 2 budget constant rather than varying it with elevation. Analysts building next-generation models should treat the Zone 2 budget as elevation-dependent, use the basin plan Zone 2 values as the best available empirical reference for conditions near 4,198 feet, and develop updated Zone 2 estimates for the low-elevation contemporary period using the Hydro Mapper gage network as that dataset matures.

The engineers who built these plans lived with a Great Salt Lake operating near its long-term equilibrium. Their documents are the closest thing available to a field manual for what the system looks like at the elevation the state is seeking to restore.

8. What is the 0.8246 inflow scaling factor, where exactly does it come from, and why is it held constant rather than recalculated each year?

The 0.8246 factor is the bridge between what the four Zone 1 gages measure and the Basin Plan estimate of total lake inflow. It converts annual four-gage terminal flow totals to estimated total lake inflow by accounting for everything that happens in the wetland transition zone between the gages and the open lake surface. This is the single most important methodological question about this framework, and it deserves a precise answer.

The four Zone 1 terminal gages—Bear River at Corinne, Weber River at Plain City, Jordan River Surplus Canal, and Jordan River at 1700 S—sum to 1,979 kaf under basin plan design conditions. The basin plans assume 2,400 kaf of total lake inflow under the same conditions. The ratio is 1,979 ÷ 2,400 = 0.8246. This framework applies that ratio to convert annual four-gage terminal flow measurements to estimated total lake inflow.

The derivation from the basin plan flow maps is as follows:

The Corinne figure of 1,232 kaf, the Plain City figure of 372 kaf, the Surplus Canal figure of 269 kaf, and the 1700 South figure of 106 kaf all appear on the basin plan flow maps reproduced in FAQ 7 and are sourced directly from the Utah Division of Water Resources basin plan documents. The derivation can be verified by reading the three basin plan water budget tables—Bear River Table 3, Weber River Table 5, and Jordan River Table 7—against the flow maps in that section.

The difference between 1,979 kaf (four-gage total) and 2,400 kaf (total lake inflow) is 421 kaf. It is a net, unresolved transition between the Zone 1 terminal-gage accounting and estimated lake inflow. It is not an independently measured or decomposed depletion category. It may reflect some combination of terminal agricultural use, wetland processes, groundwater accretion, conveyance and storage effects, the effects of mineral extraction within terminal bays, and differences in source boundaries and methods. Mineral depletion was not an explicit basin-plan delivery category. However, because part of the Compass Minerals operation lies within Bear River Bay, some historical mineral effect may be physically embedded in the net transition even though it was never separately identified. That portion cannot be isolated from the available record and is not completely separable from the approximate Mineral Depletions planning quantity used elsewhere in the framework.

The factor is held constant across all three accounting periods as a matter of methodological discipline. A time-varying factor would allow the framework to fit observed lake behavior more closely in each period, but it would do so by absorbing unexplained variance into an adjustable parameter rather than exposing it. Holding the factor constant means that residual errors in the ledger remain visible rather than being hidden inside a calibrated coefficient. If the true delivery ratio varies across hydrologic conditions—and it likely does, since wet years and dry years route water differently through the Bear River wetlands above Corinne—that variation should be quantified through independent routing analysis rather than assumed away. This framework publishes the constant factor precisely so that analysts with better routing data can propose a defensible alternative and show what it does to the conclusions.

The factor is not an invention of this framework. It is a direct numerical consequence of the basin plans' own design figures, stated explicitly here for the first time so it can be examined, challenged, and refined.

9. Which parts of this framework are direct measurements, and which involve judgment calls that a different analyst might make differently?

One of the critiques that will correctly be made of this framework is that it makes judgment calls that are presented as accounting but involve modeling assumptions. That critique is fair and deserves a precise answer.

The following components are directly derived from observed data with no modeling assumption:

The USGS gage records at the four Zone 1 terminal gages are direct measurements. The Casey-Root bathymetry is an empirically derived relationship between elevation and volume specific to the GSL system. The NOAA KSLC climate record is direct measurement. The beginning-of-water-year elevation record is direct USGS measurement. The 1,979 kaf four-gage total and the 2,400 kaf basin plan inflow figure are read directly from basin plan maps and tables.

The following components involve judgment calls that this framework makes explicitly and that reasonable analysts might make differently:

The 0.8246 factor is held constant across all periods. As documented in FAQs 7 and 8, the true delivery ratio likely varies with hydrologic conditions and lake elevation. Holding it constant is a methodological choice that preserves visibility of residual variance at the cost of period-specific accuracy.

Evaporation is the closing residual. The storage changes table closes by construction because evaporation is set equal to whatever is needed to make the ledger balance. This means the table cannot independently validate evaporation—it can only confirm that the other measured components are mutually consistent. The KSLC cross-validation (average ratio 1.011 over 23 years) provides empirical support for the consistency of those other components but does not independently confirm the evaporation estimate.

The historical 3% groundwater sensitivity. In the exploratory Hydro Mapper reconstruction, groundwater is estimated as 3% of total inflow including groundwater: 

(Lake Precip + Streamflow) × 0.03 ÷ 0.97

This is a GSL Accounting transformation of the historical whole-lake estimate of approximately 75 kaf/year, not a direct measurement or the original annual USGS calculation method. It is a simplifying assumption consistent with prior GSL water-balance studies. Recent research suggests 5–6% may be more accurate. Because groundwater is offset by the evaporation residual in this ledger structure, changing the assumption does not change any policy-relevant conclusion. It does, however, affect interpretation of the evaporation line and should be noted as a known underestimate pending better measurement.

The 300 kaf flow gap. The structural flow gap between the inflow required to maintain 4,198 ft and the current 30-year rolling inflow is derived from the ledger's equilibrium conditions at those elevations. It involves assumptions about evaporation at 4,198 ft, direct precipitation at that elevation, and the relationship between inflow and equilibrium that are based on the basin plan framework rather than direct contemporary measurement. It is the most model-dependent number in the framework. The 300 kaf figure should be understood as indicating that the required inflow deficit is in the range of several hundred thousand acre-feet per year—not that it is precisely 300 kaf. A defensible alternative set of assumptions could produce a figure of 200 or 400 kaf without being wrong. The policy implication—that the gap is large relative to any realistic conservation program—is robust across that range.

The KSLC proxy. The KSLC station is used as a spatial proxy for precipitation on the lake surface and for basin-scale ET demand. It is a single point approximately 12 miles from the nearest lake shoreline. The 1.011 cross-validation ratio demonstrates temporal consistency but does not validate spatial accuracy. Wet years that deposit precipitation preferentially on the lake surface rather than at the airport will cause the framework to understate lake precipitation and therefore understate the evaporation residual. The reverse is true for dry years where precipitation falls inland but not on the lake. The net directional bias across the full record is unknown but is unlikely to be systematic in one direction given the range of storm track patterns over the GSL basin. The 1.011 ratio—very close to 1.000 over 23 years—suggests the biases approximately cancel at the decadal scale even if individual years carry meaningful spatial error.

The North Arm elevation offset. The North Arm is modeled one foot below the South Arm throughout the historical record prior to the availability of consistent North Arm gage data, consistent with the convention used in the basin plans and in historical GSL accounting. The actual North Arm elevation has varied considerably relative to the South Arm depending on berm management and causeway permeability. This introduces uncertainty in total lake volume calculations that is not captured in the framework's stated figures.

The boundary between accounting and modeling in this framework is therefore: the conclusions about what the four-gage terminal flow was, what the South Arm elevation was, and what the climate was in each period are accounting—they come directly from measurement. The conclusions about what those measurements imply for total lake inflow, for evaporative balance, and for the feasibility of policy targets involve the modeling assumptions listed above. Both types of conclusions are presented on this site, but they carry different epistemic weight and should be cited accordingly.

10. How are lake evaporation and direct lake precipitation validated in this framework—and what are the limits of using a single land-based weather station as a reference for both unmeasurable atmospheric terms on a large saline lake?

The Great Salt Lake annual water balance closes in compatible volume units:

Ending Storage = Beginning Storage + Net Inflow + Lake Precipitation − Lake Evaporation − Mineral Depletions

where:

Net Inflow = Zone 1 Terminal Inflows / 0.8246

Beginning and ending lake elevations are converted to storage volume through the Casey-Root elevation-area-volume relationship. After the balance is closed in volume units, ending storage may be converted back to lake elevation.

The 0.8246 scaling relationship represents the net effect of the Zone 2 transition between the terminal gages and the open lake. Those processes are therefore not subtracted again as a separate, undefined “Zone 2 Depletions” line item. Summary-level diagnostic tables on the Data & Datasets page may combine the lake-evaporation residual and separately identified terminal losses into Evap & Loss; that presentation is an aggregation of the canonical identity, not a change in its accounting treatment.

Beginning and ending elevations are directly measured and converted to storage through Casey-Root bathymetry. Zone 1 terminal inflows are directly measured and then converted to estimated Net Inflow using the documented 0.8246 scaling relationship. Lake precipitation and Mineral Depletions are estimated from documented sources. Lake Evaporation is the residual required to close the annual storage balance.

That leaves two terms—Precipitation on Lake Surface and Evaporation—that cannot be directly and independently measured across a lake covering 932 to 1,595 square miles depending on elevation. They never will be, given the lake's size and the physical diversity of its surface. No rain gauge network measures precipitation uniformly across that area. No instrument measures evaporation from a saline lake of this size and physical diversity in real time. What exists instead are point measurements at weather stations near the lake, gridded satellite observations, modeled ET estimates, and indirect validation through water balance closure.

This is the standard structure of any large terminal lake water balance—and of any accounting system where certain line items cannot be directly observed and must be inferred as residuals. In financial accounting the analog is straightforward: cash is directly measured, revenues and most expenses are directly measured, but certain accruals and provisions are inferred as the residual required to make the balance sheet close. The question is not whether residuals exist—they always do—but whether the measured components are reliable enough that the residual is meaningful rather than a repository for accumulated error.

Because the equation must close, Evaporation is set equal to whatever value makes it balance—it is the closing residual. Precipitation on the lake surface cannot be independently verified either, though it enters as a measured input rather than a residual. Both terms share the same measurement limitation: a single point station cannot represent conditions across a surface that large. The KSLC station at Salt Lake City International Airport, approximately 12 miles from the nearest lake shoreline, is the reference for both.

What KSLC provides for both terms:

For precipitation: KSLC records annual precipitation that is well-correlated with regional precipitation patterns across the GSL basin. When KSLC records an anomalously dry year, the lake surface almost certainly received below-average direct precipitation. When KSLC records a wet year, the lake surface received above-average precipitation. The direction of the signal is reliable; the magnitude applied to the full lake surface area carries spatial uncertainty. The baseline ledger uses 1,400 kaf of direct lake precipitation at ~4,200 ft surface area—derived from the KSLC 16.22 in/year average scaled to lake area. This is consistent with the basin plan design values and is the best available estimate for the full record.

For evaporation: the KSLC Hargreaves-Samani ET series—computed from daily temperature, temperature range, and estimated solar radiation at KSLC—provides a temporally consistent index of atmospheric ET demand. When multiplied by total lake area from Casey-Root bathymetry at average annual elevation, it tracks the forced evaporation residual in the annual storage changes table at an average ratio of 1.011 over 23 years (2003–2025), with a standard deviation of 0.080. The 1.011 ratio is not a claim that KSLC perfectly measures lake evaporation in any given year. It is evidence that a consistent, reproducible ET series provides a stable temporal reference across a wide range of hydrologic conditions.

Hydrologists will immediately ask: how can a simplified ET model like Hargreaves-Samani, applied at a land station 12 miles from a large saline lake, produce a ratio this close to 1.000 over 23 years? The answer is that it doesn't need to produce accurate absolute evaporation estimates in any given year—it needs to track the dominant driver of year-to-year variability, which is temperature and available energy. The Great Salt Lake's evaporation is primarily energy-limited in most years. A temperature-based proxy captures that dominant signal. The salinity effect on evaporation—approximately 6–8% reduction relative to freshwater—is absorbed into the 1.011 ratio as a systematic offset. The spatial error from using a land station introduces noise but not systematic bias at the decadal scale, as confirmed by the 1.011 central tendency across widely varying storm track patterns.

What the 1.011 cross-validation proves: the framework's measured components—the four Zone 1 terminal gage flows, the USGS elevation record, the KSLC climate series, the Casey-Root bathymetry, and the 0.8246 inflow factor—are mutually consistent when applied together. A framework with systematic errors in any of these components would not produce a 1.011 central tendency over 23 years of widely varying conditions.

What it does not prove: it does not validate the evaporation estimate in absolute terms, because evaporation is the closing residual. It does not confirm that the 0.8246 factor is correct—a compensating error in the factor and in the groundwater assumption could produce the same ratio. It does not validate the spatial accuracy of the KSLC proxy for any individual year. And it does not resolve the internal split between the evaporation and precipitation terms—both are atmospheric, both are spatially uncertain, and only their net effect on the residual is cross-validated.

The 2005 outlier (ratio 0.775) and the 2018 outlier (ratio 1.143) are the expected behavior of Hargreaves-Samani in cool-wet and hot-dry extremes. These are method-consistent outliers, not framework errors.

Eddy covariance stations now being funded by the state will eventually provide direct evaporation flux measurements at specific lake locations. These will be valuable for calibrating the KSLC proxy in extreme years but will not provide the historical record length needed for long-period trend analysis. The appropriate future use is to derive period-specific correction factors that reduce the 0.080 standard deviation without changing the 1.011 central tendency.

The KSLC data series underlying both the precipitation and evaporation terms is published on the Datasets page. FAQ 11 uses these same datasets to ask the natural follow-on question: given that both atmospheric terms shift with climate, what is the net impact of the baseline-to-rolling climate shift on the inflow required to sustain the lake at any given elevation?

11. What does the shift from baseline to 30-year rolling climate imply for the inflow required to sustain the lake—and what does the accounting data tell us about that shift?

FAQ 10 establishes that direct precipitation and evaporation on the lake surface are both inferred from KSLC data rather than directly measured—and that KSLC provides a reliable temporal reference for both. That naturally leads to an accounting question that the available data can answer: how much has the shift from the 1961–1990 baseline climate to the 1996–2025 rolling climate changed the inflow required to sustain the lake at any given elevation?

In management accounting this is a standard cost structure analysis. A management accountant closing a historical ledger routinely asks: given that our input costs have shifted between the reference period and the current period, what revenue does the business now need to cover its operating costs? That analysis uses the historical ledger data as inputs and a straightforward scaling step as the bridge—it does not constitute financial modeling, it constitutes informed management accounting. The same logic applies here. The lake's atmospheric "costs"—evaporation and the offset from direct precipitation—have shifted between the baseline and rolling climate periods. The accounting data tells us exactly how much, and the question of what that implies for required inflow follows directly.

Two things shifted between the baseline (1961–1990) and the rolling period (1996–2025):

Precipitation declined. KSLC annual precipitation fell from 16.22 inches to 15.15 inches—a 6.6% reduction. Applied to a lake surface of 932 to 1,595 square miles depending on elevation, this directly reduces the precipitation term in the water balance by a measurable amount.

Evaporation increased. KSLC mean annual temperature rose from 52.2°F to 54.6°F—a 2.4°F increase. The Hargreaves-Samani ET calculation—which already uses daily temperature, daily temperature range, and calculated solar radiation—scales predictably with mean temperature. The ratio between rolling and baseline period ET is 1.046, derived directly from the period-average temperature values in the KSLC annual dataset.

Both shifts move in the same direction: less precipitation on the lake surface and more evaporation from it means more inflow is required to sustain any given elevation under the rolling climate than under the baseline climate. The net effect at any elevation can be calculated directly from the KSLC data and Casey-Root bathymetry—no external model required beyond the accounting identity already used in the ledger.

The following table presents this calculation across the 4,190–4,200 ft management range. The precipitation columns are direct accounting—KSLC inches times Casey-Root acres. The evaporation columns apply the KSLC temperature-derived HS scaling ratio to the baseline evaporation residual. The climate premium column is the net result. As in all sensitivity analysis in management accounting, the premium is labeled as an estimate derived from the accounting data—not a measured value—and the methodology is fully transparent and reproducible.

Four results from this table deserve explicit statement.

The baseline column validates the framework. The required inflow at 4,200 ft under baseline climate (2,420 kaf/year) matches the basin plan design inflow of 2,400 kaf/year to within rounding. This is not a coincidence—it confirms that the KSLC precipitation and Casey-Root bathymetry, when applied through the water balance identity, reproduce the basin plan result. The accounting closes.

The rolling column helps explain the observed baseline-to-rolling elevation decline. At 4,198 ft, the calculated atmospheric shift increases required flow from approximately 2,167 kaf/year under baseline climate to approximately 2,394 kaf/year under rolling climate—a climate difference of approximately 225 kaf/year. This difference is used as an accounting attribution for the observed baseline-to-rolling ride-down. It is not added as a separate premium to the framework’s approximately 300 kaf/year gap-to-close estimate, because the rolling-period inflow and observed elevation already embody the historical climate shift. Adding it again would count the same baseline-to-rolling climate effect twice. The central gap-to-close calculation remains the rounded comparison between approximately 2,200 kaf/year associated with 4,198 ft under the baseline equilibrium relationship and approximately 1,900 kaf/year of current 30-year Net Inflow.

Precipitation and evaporation decline as the lake shrinks. Both atmospheric terms decline in volumetric units as lake surface area contracts. Near the shock-period average elevation of approximately 4,191.9 ft, total lake area is approximately 1,000 square miles, compared with approximately 1,595 square miles at 4,200 ft. The sensitivity table applies the Casey–Root area associated with each target elevation while holding each period’s climate assumptions consistent. Required inflow therefore declines at lower elevations because the smaller lake presents less surface area for both precipitation and evaporation. That lower requirement describes equilibrium for a smaller lake; it does not represent recovery.

The estimated climate effect grows with elevation. The baseline-to-rolling difference is approximately 150 kaf/year near 4,190 ft and approximately 255 kaf/year at 4,200 ft because the atmospheric terms operate over a larger surface area at higher elevations. This sensitivity helps explain the observed elevation shift between the baseline and rolling periods. It is not a separate forward requirement to be added to the approximately 300 kaf/year accounting gap. The rolling ledger, rolling average elevation, and current Net Inflow already reflect the historical climate transition analyzed here. The more recent five-year shock regime is a separate concern. If its substantially lower inflow and warmer, drier conditions persist, the flow needed for recovery could exceed the central 30-year accounting estimate. The ledger documents that risk but does not forecast whether the shock regime will persist or assign it a new central gap-to-close value.

Relationship to the baseline-to-rolling elevation decline. The observed difference between the 1961–1990 average South Arm elevation (4,199.6 ft) and the 1996–2025 rolling average (4,195.9 ft) is approximately 3.7 feet. The climate sensitivity analysis above indicates that the shift in precipitation and evaporation accounts for roughly half of that difference. The remaining residual—approximately 2 feet—is therefore interpreted in this framework as the cumulative effect of long-term depletion growth. This is an accounting attribution based on the historical record, not a direct physical measurement.

The climate dataset underlying this table—KSLC daily observations from 1961 through 2025—is published on the Datasets page. The calculation is fully reproducible from those datasets and the Casey-Root bathymetry.

12. Why does the Bear River dominate Great Salt Lake recovery potential, and how should Idaho and Wyoming depletions be handled in a Utah policy context?

The Bear River is the single largest inflow source to Great Salt Lake. At the Corinne gage it delivers roughly 1,232 kaf under average design conditions—about 62% of the 1,979 kaf combined total across the four Zone 1 terminal gages, and roughly half of total estimated lake inflow once Zone 2 wetland and groundwater contributions are added—and dramatically more in wet years. In water year 1983 it delivered 2,890 kaf. In water years 1985 and 1986 it delivered 2,237 and 3,099 kaf respectively. The 1982–1986 sequence that produced the historic lake high of 4,210 ft was almost entirely a Bear River event. No other basin has demonstrated remotely comparable flood-year capacity.

This asymmetry has two important implications for policy. First, the lake's recovery potential in any given wet year depends disproportionately on Bear River precipitation and snowpack in Idaho, Wyoming, and northern Utah—not on Wasatch Front conditions that drive Jordan and Weber flows. A wet year that saturates the Wasatch but leaves the Bear River headwaters dry will produce much less lake response than the reverse. Second, because Bear River basin precipitation is the primary recovery mechanism, conservation actions in the Bear River basin—particularly near-lake actions with high bankability—have disproportionately high value per unit of water delivered compared to actions in the Weber or Jordan basins.

The Idaho and Wyoming component requires careful handling. The Strike Team reports, Strategic Plan, and integrated water budget analyses take a holistic basin view that includes the full Bear River system from its headwaters in Wyoming through Idaho and into Utah. Idaho Bear River agricultural depletions (approximately 571 kaf) and Wyoming Bear River agricultural depletions (approximately 113 kaf) are included in the broad-basin depletion total of approximately 1,551 kaf for agriculture under that convention. These are real depletions that reduce the flow arriving at the Utah state line—they are already reflected in what the Corinne gage measures. But they are outside Utah’s direct policy jurisdiction, they are not governed by Utah state instruments, and the Bear River Compact controls their allocation in ways that constrain Utah’s ability to affect them through state policy. The baseline Water Ledger on this site uses the Utah-only convention consistent with the basin plans; the broader GSL basin convention including Idaho and Wyoming appears in the depletion datasets and bankability tables where the full system context is relevant.

The practical consequence for policy analysis: if Idaho and Wyoming Bear River depletions are included in the depletion total used to size conservation requirements, the headline depletion number and therefore the apparent conservation potential is substantially inflated relative to what Utah policy can achieve. Approximately 684 kaf of the 1,551 kaf agricultural depletion total is in Idaho and Wyoming. A conservation analysis that treats the full 1,551 kaf as available for Utah policy action is overstating Utah's management flexibility by roughly 44%. This framework includes Idaho and Wyoming figures in the full basin accounting for completeness but explicitly flags them as outside the Utah policy action boundary when used for bankability and conservation target analysis.

The Bear Zone 2 complication: bankability to Corinne is not bankability to the lake

The opening paragraphs of this FAQ establish Bear River as the dominant lever for Zone 1 terminal inflow. That conclusion requires an important qualification: Bear River's Zone 2 is the most actively consumed of the three terminal basins, and that consumption introduces a bankability gap between water delivered to Corinne and water delivered to the open lake.

The lower Bear corridor (Idaho-Utah State Line to Corinne) averages +448 kaf of tributary amplification over the 1971–2025 record—driven primarily by Logan River, Blacksmith Fork, and Cache Valley springs. This amplification is a genuine contribution to Zone 1 flow. But once that water passes Corinne, it enters a Zone 2 system that under drought conditions operates in maximum-absorption mode. The Zone 2 managed depletions alone—175 kaf from State and Private Wildlife Areas and the Bear River Migratory Bird Refuge—consume a floor of water regardless of lake elevation or inflow volume. Above that floor, the weir gates of the BRMBR are operated to optimize waterfowl nesting habitat, which means retaining water in managed cells rather than passing it to the open lake.

The quantitative marker for this gap is the shock period data. In 2021—a drought onset year—the lower Bear corridor delivered only +26 kaf of amplification above the State Line (design: +528 kaf), and Zone 2 managed consumption of 175 kaf would imply negative net delivery to the open lake from the lower Bear system alone. The corridor was in maximum deficit mode simultaneously at Zone 1 and Zone 2.

The approximately 200 kaf threshold introduced in FAQ 6 is the Bear-specific number: sustained annual Corinne flow above Zone 2 managed consumption of 175 kaf plus recharge deficit service—approximately 200 kaf above design Zone 2 depletions, held for multiple consecutive years—is the condition under which Bear River shepherding actions begin producing bankable open-lake inflow. Below this threshold, Bear shepherding delivers real wetland and wildlife value but not measurable lake elevation.

This is not a reason to avoid Bear River shepherding programs. The BRMBR is one of the most productive waterfowl habitats in North America and its water needs are legitimate and important. But it is a reason to evaluate Bear River shepherding proposals against the full pathway from Zone 1 delivery to Zone 2 throughput to open-lake elevation, rather than treating Corinne delivery as a proxy for lake benefit. A shepherding program that delivers 100 kaf to Corinne in a drought year may produce zero measurable open-lake elevation gain while producing substantial wildlife habitat value—both outcomes are real, but they are different outcomes, and policy proposals that conflate them will consistently overpromise on lake recovery while undervaluing the habitat benefit that is actually being delivered.

13. Why do depletions in the Utah Lake sub-basin require special treatment in both the ledger and bankability analysis?

Utah Lake sits between the majority of the Jordan River basin's water supply and the Great Salt Lake. It is simultaneously a major water body, a managed reservoir, an agricultural supply source, a receiving body for municipal wastewater returns, and the upstream boundary of the Jordan River corridor that delivers water to the lake. This complexity makes Utah Lake the single most analytically difficult component of the GSL water balance, and it is the place where depletion figures are most likely to mislead policy analysis if taken at face value.

The ledger problem: why Utah Lake depletions cannot simply be added to available supply

The basin plan accounting framework treats the Jordan Narrows—the outflow point from Utah Lake into the Jordan River—as the effective upstream boundary of the Jordan River GSL accounting system. The basin plan specifies 295 kaf at Jordan Narrows as the design delivery to the Jordan River corridor under average conditions. That 295 kaf is what appears in the available supply side of the GSL ledger for the Jordan basin.

Utah Lake itself has its own internal water budget, visible in the Jordan River Basin Plan Table 6: approximately 561 kaf of surface and groundwater inflow, 201 kaf of open-lake evaporation, 52 kaf of withdrawals for irrigation and M&I, and 308 kaf of outflow at Jordan Narrows (the 295 kaf basin plan figure is a slightly different time-period average of the same flow). The 201 kaf of Utah Lake evaporation is an enormous consumptive loss from a relatively small, very shallow lake—Utah Lake’s shallow bathymetry produces extremely high evaporation per unit area. This evaporation is not a human depletion but it is a massive natural tax on any water that enters Utah Lake before flowing north to GSL. The Utah Lake M&I depletion figure varies substantially depending on boundary convention: the broader Lower Jordan Basin convention used in the basin plans and Strike Team reporting includes Salt Lake County M&I use throughout the Jordan corridor, producing figures in the range of 181–257 kaf; a narrower convention limited to Utah County and Utah Lake’s immediate service area is substantially lower. This boundary sensitivity is part of the broader depletion figure variation documented in FAQ 14.

The implication for ledger construction: Utah Lake agricultural depletions (approximately 145 kaf) and Utah Lake M&I depletions are consumptive uses that reduce the volume of water flowing out at Jordan Narrows—the outflow point specified at 295 kaf in the Jordan River Basin Plan Table 7, already cited in FAQ 7. They are correctly included in total basin depletion figures when the question is "how much water is consumed in the GSL watershed." But they should not be added to the GSL ledger on the available supply side above the Jordan Narrows line unless the Jordan Narrows delivery figure is simultaneously reduced to remove the return flow credit already embedded in the 295 kaf estimate. The basin plan's 295 kaf already reflects the net of Utah Lake inflows, depletions, evaporation, and outflow. Double-counting Utah Lake depletions on both the depletion side and the available supply side without adjusting the Narrows figure will cause the ledger to not close.

The bankability problem: why Utah Lake conservation savings are worth less to GSL than the depletion figures suggest

Even setting aside the ledger construction issue, the bankable value of Utah Lake agricultural or M&I conservation for the Great Salt Lake is substantially lower than the gross depletion figures suggest for three compounding reasons.

First, Utah Lake evaporation. Any water saved in the Utah Lake basin that is not immediately routed out the Jordan River corridor will accumulate in Utah Lake, where it is subject to the 201 kaf evaporative loss from that shallow water body. Water that pools in Utah Lake waiting for delivery opportunity does not wait patiently—it evaporates at a rate that makes Utah Lake one of the least efficient water storage bodies in the state per unit of surface area.

Second, Jordan River channel constraints. The Jordan River corridor through Salt Lake County is a managed urban channel with fixed flood stage infrastructure. The basin plan delivers 269 kaf at Surplus Canal and 106 kaf at 1700 South under design conditions. The channel's capacity to pass significantly more water than design flow in any given year is limited by flood management requirements in a densely urbanized watershed. This means that even if water is freed up at Utah Lake, the physical pathway to GSL is constrained. Unlike the Bear River, which has demonstrated the ability to deliver 3,656 kaf in a single exceptional year (water year 1984—2.97 times its basin plan design value), the Jordan system operates in a relatively narrow band around its design delivery. Conservation savings in the Utah Lake basin that cannot be routed through the Jordan corridor in the year they occur will be held in reservoir storage or lost to evaporation. The Jordan system's structural stability relative to design is confirmed by the Zone 1 gage data: Jordan terminal gages averaged 387 kaf over the 1991–2020 rolling period against a design of 375 kaf—103% of design—even as Bear River ran at 74% of design and Weber at 67%. Utah Lake's buffering function produces that stability, but the same buffering function captures and evaporates any water that would otherwise result from upstream conservation actions.

Third, return flow dependency. A significant portion of the Jordan River's design delivery to GSL depends on wastewater treatment plant returns cycling through the system. The general principle is that M&I water use that returns to the river as treated effluent is already credited in the terminal gage measurements—it is not consumed, it is delayed and returned. SLCWWTP contributes approximately 41 kaf directly to the Jordan River corridor. MWWTP contributes approximately 2.4 kaf via the Effluent Ditch. These return flows are already embedded in the 295 kaf Jordan Narrows figure and in the terminal gage measurements. M&I conservation in the Salt Lake County area that reduces indoor water use will reduce these return flows proportionally, partially offsetting the conservation benefit at the GSL. This is why M&I indoor conservation carries near-zero bankability basin-wide—the return flow that would have reached the lake is removed along with the consumption it offsets.

What this means for policy

Utah Lake basin depletions should be included in total basin accounting for completeness and for understanding the full scale of human consumptive use in the watershed. This framework supports that inclusion. But they require a clear position in the ledger—they are upstream of the Jordan Narrows accounting boundary that the basin plan uses, and any conservation credit claimed for them must be adjusted for the Jordan channel routing constraints and Utah Lake evaporative losses before appearing as bankable GSL benefit.

The practical consequence: a policy proposal that identifies 145 kaf of Utah Lake agricultural savings and presents that as 145 kaf of bankable GSL inflow is overstating the benefit by a factor that could range from 2 to 10 depending on hydrologic conditions, channel capacity, and evaporative timing. The low end of that range applies in wet years when Utah Lake is spilling and the Jordan corridor has capacity to pass additional flow—conditions under which conservation savings move efficiently to the lake with relatively modest evaporative losses. The high end applies in dry years when Utah Lake is below spill threshold, any saved water pools in Utah Lake and evaporates, and the Jordan corridor is already operating below capacity. The bankability analysis required to properly evaluate Utah Lake conservation proposals is more complex than for Bear River basin actions and requires routing analysis that goes beyond what this framework's ledger structure can directly provide. This framework flags the issue and recommends that any Utah Lake conservation proposal be evaluated with an explicit bankability calculation rather than a gross-depletion substitution.

14. Why do total depletion figures vary substantially across official state reports—and is that variation a sign of disagreement or of methodological evolution?

Depletion estimates have varied substantially across official sources over the past two decades—not primarily because underlying water use changed, but because definitions, boundaries, measurement methods, and classification conventions changed without explicit documentation of what changed and why.

The table below shows a 30-year period average from each major official report, computed from the period-band data published in each report and chosen to approximate the 1991–2020 window as closely as each report's banding structure permits. This is the most apples-to-apples comparison available across reports. The columns are still not directly comparable without the boundary and methodology adjustments described in the notes.

The principal sources of variation are as follows.

The Basin Plans are design-year forecasts, not measured averages. This is the most important framing distinction in the table. The Basin Plans columns—Bear River Basin Plan Table 3, Weber River Basin Plan Table 5, Jordan River Basin Plan Table 7—represent what planners expected consumptive use to be under long-term average hydrologic conditions at the time the plans were written (~1990-2016). They are forward-looking planning constructs anchored to 1961–1990 hydrology and contemporary depletions as measured at that time. They are included here to show the planning foundation this framework builds on, not as a measured historical average. The low Agriculture figure (728 kaf) reflects Utah-only depletions under the narrower basin convention and the measurement methods available in 1990, not lower actual use.

Geographic boundary—the most important source of incomparability across measured columns. The 2021 Water Plan column reflects Utah-only Bear, Weber, Jordan, and Utah Lake basin depletions but excludes Idaho and Wyoming Bear River agricultural depletions. The 2023–2026 Strike Team columns include Idaho and Wyoming Bear River depletions (approximately 684 kaf agriculture combined) under the broader GSL basin convention. This single boundary difference accounts for most of the Agriculture increase from ~882 kaf (2021 Water Plan) to ~1,334–1,506 kaf (Strike Team columns)—it is not primarily a change in actual Utah water use. Any analysis that treats this jump as representing increased Utah consumptive use has made a boundary error.

No column in this table uses the same geographic boundary as the Baseline Water Ledger on the Water Ledger page, which covers Utah Bear, Weber, and Jordan basins only and excludes both Idaho/Wyoming and Utah Lake. Analysts comparing this table to the Water Ledger must adjust for these boundary differences before drawing conclusions about trends in actual water use.

The bankability implication of boundary expansion. The state's position that the full GSL basin—including Idaho, Wyoming, and Utah Lake—gives the most complete picture of total consumptive use in the watershed is correct and this framework does not dispute it. The complete basin convention is the right foundation for understanding the full scale of human water use that affects the lake. The bankability caveat is different from and does not contradict that position. It is this: a depletion figure that includes Idaho and Wyoming Bear River agriculture (~684 kaf) and Utah Lake depletions represents water use that Utah policy instruments cannot directly reduce. Idaho and Wyoming depletions are governed by the Bear River Compact and are outside Utah's administrative jurisdiction. Utah Lake depletions face the routing and evaporation constraints documented in FAQ 13. When a depletion total grows because the boundary expanded to include these components, the apparent conservation potential—the gap between current depletions and some lower target—grows by the same amount, but the bankable share of that gap does not. A policymaker who reads a headline depletion total of 2,300+ kaf and concludes that Utah conservation programs can close a correspondingly large gap is working from a number that includes approximately 800–900 kaf of depletions that Utah programs cannot reach at all or cannot reach efficiently. The bankability lens does not challenge the completeness of the full-basin accounting. It asks the follow-on question that full-basin accounting alone does not answer: of the total depletion figure, how much is bankable as Great Salt Lake inflow through actions available to Utah policymakers, and on what timeline?

The 2021 Water Plan column shows depletions, not diversions. The 2021 Utah Water Plan reports both diversions and depletions by basin. The column above uses the depletion figures (M&I 349, Agriculture 882) based on the implicit return flow percentages published in that report—M&I approximately 47% statewide return, Agriculture approximately 61% statewide return, derived from the diversion and depletion paired figures in the 2021 Utah Water Plan Table 3-2. The diversion figures (M&I 746, Agriculture 1,440) are substantially higher. An analyst who uses the 2021 Water Plan diversion figures as depletion inputs to a water balance will substantially overstate consumptive use.

M&I outdoor depletion factor revision—the largest single methodological change. The 2026 Strike Team M&I figure (~628 kaf, 30-year average) reflects a revision of the outdoor depletion factor from 40% to 91% of outdoor water use, documented in the Utah Water Budget Handout (updated December 2025). All prior report columns use the old 40% factor. This single change drives the M&I increase from the 2024 Strike Team Data and Insights figure (~334 kaf, old factor) to the 2026 Strike Team figure (~628 kaf, new factor)—a near-doubling that reflects improved measurement of consumption that was always occurring, not a change in actual water use. The 2026 Strike Team acknowledges this as the primary driver of M&I rising from 16.4% to 26.3% of total depletions.

Wetlands treatment. The Basin Plans included 665 kaf for managed wetland ET treated as a depletion—primarily the Bear River Bay and Farmington Bay wetland systems. The 2023–2024 Strike Team reports carried this at 175–181 kaf under the label Managed Wetlands. The 2025–2026 Strike Team reports omit the Wetlands line entirely, redistributing that consumption into natural system ET or treating it as outside the depletion boundary. This treatment is the primary reason the Basin Plans total (~1,682 kaf) appears lower than the Strike Team totals despite covering a smaller geographic area—the 665 kaf wetlands line in the Basin Plans represents a significant depletion category that later reports handle differently.

New line items appearing in later reports. Ag Incidental (34–36 kaf) appears from the 2024 Data and Insights report onward—riparian vegetation adjacent to canals and flood-irrigated fields, previously subsumed in other categories. Reservoir evaporation (22–24 kaf) appears from the 2023 Strike Team onward for the same reason. These are not new depletions but previously subsumed items given their own lines as measurement improved.

Period averaging and band-shift effects. The 30-year averages in the table above are computed by this framework from the period-band data published in each report, not taken directly from a published summary row. The five-year bands shift between reports—for example, the 2023 Strike Team bands cover 1991–2020 while the 2024 Policy Assessment bands cover 1991–2021 and the 2026 Strike Team bands cover 1990–2024. The band-shift effect on the 30-year average is small and does not materially affect cross-column trend analysis. The specific bands used for each column's average are stated in the column header row of the table above.

A note on versioning

The variation documented here is not a criticism of the agencies involved. Improving measurement, refining boundaries, and reclassifying categories as understanding improves is exactly what good water accounting should do. The Utah Water Budget and recent Strike Team reports have significantly improved transparency by documenting assumptions, methods, and category definitions more explicitly than earlier efforts.

The gap discussed here is different. It concerns continuity across reporting cycles. When a depletion figure changes materially between reports, the change should ideally be accompanied by a concise explanation identifying what drove the difference—boundary revision, ET methodology update, secondary water reclassification, revised depletion factors, or a new accounting line item. This is standard practice in financial reporting, where material restatements are accompanied by reconciliation notes explaining the change from prior versions. This framework commits to that standard for its own outputs and encourages the same approach wherever long-term water accounting datasets are maintained.

15. What does the post-1987 structural decline in terminal streamflow mean for Great Salt Lake accounting and recovery expectations?

One purpose of accounting is to reveal patterns that are difficult to see when datasets are viewed in isolation. This framework was not originally developed to investigate long-term streamflow regime changes. It was developed to reconcile basin plans, terminal gages, depletions, climate, and lake response within a common accounting structure. In doing so, it surfaced an observation that deserves careful attention.

What the four gages show

The four Zone 1 terminal gages—Bear River at Corinne, Weber River near Plain City, Jordan River Surplus Canal, and Jordan River at 1700 South—form the backbone of this framework. Together they measure the water leaving the three major tributary systems before it enters Bear River Bay, Farmington Bay, and the broader wetland transition zone on its way to the open lake.

Viewed together over the full 1961–2025 record, these gages reveal a pattern that is difficult to see when each basin is analyzed independently: all three major tributary systems appear to operate differently before and after the mid-1980s. The observation is most pronounced on the Bear River, which supplies the largest share of Great Salt Lake inflow, but it is visible to varying degrees across the Weber and Jordan systems as well.

The purpose of this figure is not to prove a cause. It is to document the observation clearly. Any explanation—whether climate variability, long-term climate change, changing snowpack efficiency, basin development, measurement differences, or some combination of factors—must ultimately be consistent with the record shown here.

The pattern is most visible on the Bear River at Corinne, which contributes roughly 60% of Great Salt Lake terminal inflow. Between 1983 and 1986, Bear River at Corinne delivered the four largest annual flows in the modern instrumental record: 2,890 kaf (1983), 3,656 kaf (1984), 2,237 kaf (1985), and 3,099 kaf (1986)—averaging 2,971 kaf, or 2.41 times the basin plan design value of 1,232 kaf. Those four years drove the lake to its highest recorded elevation and triggered construction of the West Desert Pumping Project.

Then the pattern changed.

In 1988, Bear River at Corinne delivered 525 kaf. In 1989, 601 kaf. In 1990, 456 kaf.

In the 38 years since 1987, only five years have exceeded 1,500 kaf: 1997, 1998, 1999, 2011, and 2017. The entire 2001–2010 decade averaged just 694 kaf—lower than the 2021–2025 crisis period average of 769 kaf. By comparison, the 1964–1990 baseline averaged 1,429 kaf. Bear River has not sustained that level in the 38 years since.

The same directional change is visible in Weber River flow at Plain City and in the Jordan River Surplus Canal record. Only Jordan at 1700 South—buffered by Utah Lake—fails to exhibit a proportional response. The apparent shift crossed state lines, watershed divides, and elevation zones simultaneously. That observation alone makes it worthy of investigation.

The observation is clear. The interpretation is not.

The accounting framework records what happened. It does not claim to explain why.

A natural reaction is to mark 1987 as a precise hydrologic breakpoint because it immediately follows the 1983–1986 flood peak and is visually prominent on the charts. The framework does not assume that. The apparent transition could reflect a structural shift beginning near 1987, the end of an unusually wet period, a gradual change that began earlier, a combination of factors, or something else entirely—establishing causation requires analysis that goes beyond what a ledger can resolve.

What the framework can say is this: the apparent change predates the period of significant depletion growth in the Bear River basin. The basin plan depletion estimates—calibrated to approximately 1990 conditions—represent the depletion baseline. The gage record already looks different before that baseline was established. That timing is a piece of evidence. What it means requires further investigation.

What the existing literature suggests

This accounting observation does not stand alone. Several independent lines of peer-reviewed research provide relevant context—though none were written specifically to explain the Corinne gage record.

The 1,200-year paleoclimate record. A 2015 study by DeRose, Bekker, Wang, and colleagues reconstructed roughly 1,200 years of Bear River streamflow from tree-ring data. Their findings indicate that the latter half of the 20th century was among the wettest extended periods in the entire reconstruction, and that 1983–1986 ranks among the highest-flow episodes of the last millennium. The researchers draw an explicit parallel to the Colorado River, where infrastructure and allocation decisions were made during an anomalously wet period—a comparison that has been called one of the most consequential planning errors in western water history. The implication for Bear River is direct: the pre-1987 average at Corinne may embed conditions that are not representative of the long-run norm.

Pacific Ocean climate cycles. Wang, Gillies, Reichler, and colleagues documented that Great Salt Lake elevation responds to decadal and multidecadal oscillatory modes in the Pacific—a quasi-decadal oscillation (QDO) and the Interdecadal Pacific Oscillation (IPO). Their work shows that the strongest hydrologic responses in the Great Basin occur during transitions between climate regimes rather than during the regimes themselves, and that GSL elevation consistently lags Pacific sea surface temperature signals by roughly six years. The timing of the apparent Bear River shift is broadly consistent with a transition out of the warm Pacific phase that began in the mid-1970s.

The rain-for-snow substitution. Gillies, Wang, and Booth documented a long-term decline in the fraction of Utah winter precipitation falling as snow. Because Bear River flow depends heavily on mountain snowpack accumulation and spring snowmelt, a shift toward more rainfall and less snowfall reduces streamflow efficiency even when total precipitation is similar. Earlier runoff encounters higher evaporative demand on its way to the lake.

These three lines of evidence are convergent but not conclusive. They were developed independently of this framework and none were intended to explain the terminal gage records directly. Together they point toward plausible climate-related mechanisms. Establishing attribution with confidence requires additional analysis.

Why this matters for the accounting

The significance of this observation is not that it proves a particular cause. It is that many planning documents, policy discussions, and public narratives implicitly treat the pre-1987 flow regime as the baseline expectation—the normal condition toward which the lake should recover.

If that regime was itself unusual by longer historical standards, that assumption deserves examination.

The 4,198-foot restoration target in state law requires approximately 2,400 kaf of total lake inflow under the basin plan framework. The 1,429 kaf Bear River baseline was calibrated during a period when delivering that inflow was physically achievable. Whether similar inflows remain achievable under the apparent post-1980s regime is a legitimate question the accounting framework surfaces but cannot answer alone.

This is not a counsel of despair. Multi-year recovery periods remain possible, and the post-1987 record includes years that exceeded 1,500 kaf at Corinne. The 2023–2024 partial recovery demonstrated that the physics of refill still function when water arrives.

But the paleoclimate record, Pacific climate research, and nearly four decades of terminal gage observations are telling a consistent story. The contribution of this framework is not to settle the debate. It is to make the question visible, state it precisely, and provide the accounting structure needed to investigate it correctly.

What comes next

A complete investigation would extend beyond the Great Salt Lake basin. One natural test is whether similar apparent regime shifts appear in other western river systems—particularly the Upper Colorado River Basin, where comparable Pacific teleconnection dynamics and snowpack trends would be expected to produce similar signals if the explanation is large-scale atmospheric forcing rather than something specific to Great Salt Lake tributaries.

That analysis is planned as a future episode. The datasets underlying the four gage charts above are available on the Data & Datasets page for any researcher who wants to examine them independently.

Key takeaway

The accounting framework does not claim to know why terminal inflows appear different today than they did during the planning era. Its contribution is that it makes the question visible, defines it precisely, and provides a consistent structure for investigating possible explanations—starting with the observation that the apparent change predates the depletion growth it is often attributed to.

16. How does the statewide basin plan yield cross-validation work, and why does it produce a different total than the state's published statewide figure?

The Great Salt Lake Water Ledger notes that summing the eleven Utah basin plans produces approximately 8,481 kaf of total basin yield, compared to the 7,700 kaf shown in the state's statewide infographic. The table below documents the source for each basin's yield figure and explains the 781 kaf methodological difference. Two basins—West Desert and Kanab/Virgin—do not have a single published yield table in their respective plans; those figures are GSL Accounting estimates derived from plan components and are flagged accordingly.

This table represents GSL Accounting's synthesis of the eleven Utah basin plans published by the Utah Division of Water Resources. Nine of the eleven yield figures are taken directly from named tables in state-published documents; West Desert and Kanab/Virgin are GSL Accounting estimates flagged above. The plans span two decades of state water planning: Kanab/Virgin (1993), Cedar/Beaver (1995), Sevier (1999), Southeast Colorado (2000), West Colorado (2000), West Desert (2001), Bear River (2004), Weber (2009), Jordan (2010), Utah Lake (2014), and Uintah Basin (2016). When a plan is updated, the corresponding yield figure and source citation in the table above should be reviewed against the new document before the 8,481 kaf total is cited.

17. What is the Great Salt Lake Distribution Accounting Tool (DAT), and where does the water it tracks actually exist?

The Distribution Accounting Tool (DAT) is the Utah Division of Water Rights' administrative ledger for water that has been legally dedicated to Great Salt Lake. It is not a hydrologic model and not a physical inventory. It is a running balance—evaporation charges are updated daily, while accretion credits are updated as river commissioner streamflow reports are finalized—that tracks how much of the lake's storage should be attributed to legally dedicated water after applying the DAT's own accounting rules: crediting measured or reported accretions under approved change applications, reallocating the balance between the North and South Arms as water moves through the railroad causeway, and deducting evaporation modeled against the marginal surface area those dedications are assumed to occupy.

As of January 1, 2026, the DAT displays a cumulative balance of 476.115 kaf. That figure is the result of five years of accounting, not a single delivery: 520.538 kaf of credits were added between January 1, 2021, and December 31, 2025, offset by 44.412 kaf of modeled marginal evaporation over that period and a further 0.011 kaf on the first day of 2026.

The DAT balance should not be confused with the Utah Division of Water Resources’ January 2026 Strike Team report, which separately describes nearly 400 kaf of water being dedicated and delivered to Great Salt Lake between 2021 and 2025. The figures are related but use different reporting cutoffs. Because evaporation is charged daily while accretion credits depend on river-commissioner reports finalized after each water year closes, the DAT balance is trued up as those reports become available. The Strike Team figure reflects an earlier point-in-time estimate rather than the fully reconciled year-end DAT balance.

Where does this water physically exist? Nowhere separate. Great Salt Lake has no physical partition between dedicated and non-dedicated water—a molecule credited under a 2022 change application is indistinguishable from one that fell as precipitation in 1975. The 476.115 kaf balance is the State’s modeled accounting attribution of a portion of the lake’s current storage to legally dedicated actions after applying the DAT’s accounting rules. It is not a reservoir account, not a pool that can be called, released, or redirected, and not separable from the lake’s general water mass.

That has two practical consequences:

• The balance is legal and administrative, not a separate physical claim. There is no mechanism to withdraw “the DAT’s 476.115 kaf” independently of the lake’s overall water level because that water does not exist as a distinguishable physical quantity. The legal status still has real effect: once water is credited to the DAT, it is treated as committed to the lake and is not available for appropriation to other uses, even though it cannot be physically isolated within the lake’s total volume.
• The balance is cumulative and does not represent a single year’s benefit. It is a five-year running balance, not an annual figure, and it does not by itself show how much recurring bankable lake accretion the current portfolio is likely to produce. That separate question is addressed on the Dedicated Water Reconciliation page by applying the Great Salt Lake Accounting Standard (GSLAS) order by order. The active portfolio contains 549.456 kaf of fixed order maximums, while the available record supports quantified representative annual bankable-accretion subtotals of 158.515 kaf/year under the dry regime and 119.707 kaf/year under the wet regime. Another 193.718 kaf of fixed order maximum is associated with actions whose annual bankable benefit remains unquantified pending measured delivery, reported participation, documented depletion, or a supported counterfactual baseline. The Ontario Drain Tunnel provides an additional measured, loss-adjusted benefit without a fixed annual quantity. These annual portfolio measures are fundamentally different from the DAT’s 476.115 kaf cumulative administrative balance.

See the Dedicated Water Reconciliation page for the full order-by-order accounting behind these figures.

Great Salt Lake Dedicated Water Reconciliation

Does the DAT's evaporation charge mean it functions as a form of reservoir accounting? In a narrow, mechanical sense, yes: the DAT charges the dedicated-water balance a computed evaporation cost each year based on the marginal lake surface area that balance is modeled to occupy—the same basic logic a reservoir operator uses to track evaporative losses against a stored volume, and the same logic reflected in the DAT dashboard's own bucket graphic, which displays "Non-Dedicated Water" and "Dedicated Water" as two distinct partitions of the lake's total volume. But what enters that account is tied to measured streamflow at the point of credit, not simply the face value of an approved change application taken in isolation. For example, if 100 cubic feet per second is measured arriving at the D Line dike at the Bear River Migratory Bird Refuge, and 10 cfs of that total is attributable to an upstream donation, the accounting records 90 cfs as base flow and 10 cfs as the dedicated credit—both components drawn from and reconciled against the same gaged total, not simply asserted from the change application's authorized amount. What can still diverge from that initial measured credit is durability over time—whether the same volume continues to be measured arriving in future years and under different climate regimes—which is the question the order-by-order analysis on the Dedicated Water Reconciliation page addresses.

The dashboard’s “Non-Dedicated Water” figure—8,300 kaf as of January 1, 2026—is best understood as a derived residual: the lake’s measured total volume minus the 476.115 kaf DAT balance. It is not an independently measured separate pool because Great Salt Lake is not physically divided into dedicated and non-dedicated water. The graphic accurately represents what the DAT model does; it should not be read as a verified claim that the lake is exactly 476.115 kaf higher than it would have been without the credited actions.

A note on what this site is not

This site is not an advocacy document. It does not argue for or against any specific water rights holder, any particular conservation program, or any political position on how the Great Salt Lake should be managed. This site presents Accounting Findings derived from defined data, documented assumptions, and reproducible calculations, together with Analytical Inference concerning system drivers and policy feasibility. It does not prescribe policy outcomes. Where a conclusion extends beyond what the physical accounting directly demonstrates, the supporting assumptions and reasoning should be identified. Where the arithmetic supports conservation—Conclusion 2, which documents the chronic 2-foot depletion-driven baseline depression—this site says so directly. Where the arithmetic challenges the dominant conservation narrative—Conclusion 1, which identifies the 2021–2025 crisis as primarily climate-driven—this site says that directly too.

This site is not an alternative model competing with the state's analytical tools. The DWRe Water Budget, the USGS HydroMapper, and the Strike Team reports are the authoritative sources for their respective purposes and this framework builds on them. The contribution here is assembly, transparency, and closure—taking the inputs those tools provide and constructing a single reproducible ledger that names every assumption and shows every calculation.

What this site is: a reproducible, assumption-explicit, closed-ledger accounting framework for the Great Salt Lake water system, designed to preserve institutional knowledge at risk of generational loss, to force methodological transparency in a policy conversation that urgently needs it, and to provide the structured factual foundation that both human analysts and AI systems will need to reason accurately about the Great Salt Lake for the next generation.

Version 1 reflects the state of the data and the framework as of the 2025 water year. Version 2 will incorporate 2026 water year data when it is complete, will refine any methodology items that generate substantive technical feedback, and will expand the FAQ as questions are received. The changelog will document every substantive revision and its reason, consistent with the versioning standard this framework advocates as a standard for the official reports.

Great Salt Lake Accounting Changelog

← Conclusions | Changelog →

© 2026 GSL Accounting · Version 1.1 (August 2026). Content may be shared or cited with attribution. Built by humans with AI assistance for data validation and drafting, and grounded in public data; analytical judgments and conclusions are the authors’ own. See the Changelog for revision history and archived change notes.