Katie
Water Desk: When the West’s rivers surge each spring, older groundwater dominates the runoff
A better understanding of groundwater in the headwaters could improve streamflow predictions
By Mitch Tobin - September 2, 2025
Every spring, high-country streams and rivers in the American West begin to swell with water as the region’s snowpack starts to dissipate.
It’s easy to assume that the liquid flowing in these water bodies is just fresh meltwater emanating from the preceding winter’s snowpack.
But a recent study of 42 sites across the West finds that both the conventional wisdom and some traditional hydrologic models are wrong.
Most of that vital runoff—which sustains both ecosystems and economies—is actually groundwater that is many years old.
In the headwaters they studied, the researchers found the average age of the water in streams during snowmelt runoff was 5.7 years. Overall, about 58% of the runoff was derived from older groundwater that had been essentially pushed into the stream by the newer snowmelt.
“Contrary to the common assumption that snowmelt quickly contributes to runoff, stream flow during snowmelt in western US catchments is dominated by older groundwater,” according to the study, published in Communications Earth & Environment.
The spring and summer flows in the headwaters are certainly shaped by how much snow fell during the prior months. But the researchers argue that water managers need to account for longer periods of precipitation, the underlying geology and the resulting groundwater storage if they are to make more accurate forecasts of the runoff that supplies both natural and human communities downstream.
“We think it represents a fundamental change in how we think of water resources in the West,” said lead author Paul Brooks, a professor of hydrology in the Geology and Geophysics Department at the University of Utah.
People may think the melting snowpack “runs off the surface like spilling water on your countertop,” Brooks said, but “we know it’s more complicated than that.”
Many existing models of mountain hydrology assume that shallow soils sit on top of bedrock that lies close to the surface and doesn’t let much water through. Models also assume there’s minimal change in groundwater from year to year.
But Brooks said the study shows “without a doubt, there’s large and variable amounts of groundwater storage in the mountains.”
Implications for water management
The role of groundwater recharge and storage has important implications for how the West’s limited water supply is managed. Multiyear groundwater deficits could help explain why runoff in some basins has been far below average in recent years despite decent snowpack readings. Incorporating groundwater into runoff models could make predictions more accurate and inform how water is allocated over several years, not just season by season.
For Lake Powell and Lake Mead, the two largest reservoirs on the Colorado River and in the nation, “if you had the ability to predict several years in advance, you would change your management,” Brooks said. “If we can predict when there’s going to be more or less (water) available based on how groundwater storage responds to climate, we can look at a multi-year planning cycle rather than just starting all over again every year.”
“I’m very excited about this,” Brooks said. “It came out far better than I ever could have hoped for a sabbatical project.”
Keith Musselman, an assistant professor at the University of Colorado Boulder’s Department of Geography who wasn’t involved in the study, said that hydrologists studying riparian areas in lowland valleys have long described a “piston effect” in which newer precipitation pushes older groundwater out into a river.
“What Paul Brooks and his co-authors have done is bring that to the mountains, and mountains are important because they are really where the majority of our water in the West comes from,” Musselman said. “This paper kind of ties together a number of really important studies and highlights that there is this storage system that is important to consider when we’re thinking about, particularly, water conservation, water supply planning and, ultimately, climate change impacts on our ability to provide water to society.”
Overall, snowmelt-driven streamflow and groundwater recharge in the West’s mountain watersheds are “the primary water sources for 70 million people” across 10 states in the American West, according to the study. “This water supports municipal, industrial, agricultural, hydropower, and natural ecosystems powering a $9.8 trillion economy, trailing only the entire US and China in global economic activity.”
Dating the age of runoff
During his sabbatical, Brooks pursued what he called a “high-risk, high-reward” project. He traveled across the region to collect water samples from five major river basins that were then analyzed using tritium, a radioactive form of hydrogen.
Tritium is a hydrogen isotope that occurs naturally in small amounts, but its prevalence in the atmosphere spiked in the 1950s and 1960s due to the testing of nuclear weapons during the Cold War.
When precipitation falls, it contains tritium from the atmosphere, but the isotope decays relatively quickly, with a half-life of about 12.3 years, meaning around half of it transforms into a form of helium every dozen years or so. By measuring how much tritium remains in a water sample and comparing that to the expected level when the liquid fell as precipitation, scientists can estimate the water’s age.
“It creates a very, very precise clock,” Brooks said.
Brooks and other researchers collected samples of water in the middle of winter, when the flows were low and stable, and the tritium analysis confirmed that this baseflow was derived from groundwater storage; its age averaged 10.4 years from the time it originally fell as rain or snow.
Then the scientists gathered water samples during spring and early summer, when the snowmelt was running off, and they found that the water averaged nearly six years old, meaning that groundwater was still making a major contribution to the overall streamflow.
Importance of underlying geology
Looking across the diverse sites, the researchers concluded that the underlying geology played a key role in determining the age of the runoff. The sites with younger baseflows were underlain by hard rock and shale, which are relatively impermeable, so groundwater contributed less to the flows. Sites with older baseflows were characterized by more porous rock, such as sedimentary layers, that allowed the water to infiltrate far more effectively.
“If you have a subsurface underlying geology that has very, very low permeability, there really isn’t any place for that water to be stored,” Brooks said. “If you have high permeability and higher porosity, it’s easier to get water in and there’s more places for water to store.”
Soil moisture is another key factor that explains runoff patterns. But the authors note much more water is held in groundwater storage, and a focus on soil moisture in modeling is “also due to the fact that soils are relatively easy to measure and monitor compared to deeper groundwater.”
Brad Udall, a senior water and climate research scientist at Colorado State University’s Colorado Water Center, called the paper “eye-opening.”
“Paul and co-authors have done a really comprehensive look at a part of the water balance that heretofore has been mostly, completely understudied,” said Udall, who was not involved in the study. “It turns out this part of the water balance is looking to be more and more important in a changing climate world.”
With other studies highlighting the depletion of underground aquifers in the West, the paper’s focus on groundwater has “some pretty scary implications for future runoff in the Colorado and other Southwestern rivers,” Udall said.
“The Southwest’s water cycle has been flashing warning signs for 26 years . . . and this is just yet one more big warning to pay attention to,” he said, noting that much more research is needed on the West’s groundwater.
“What really . . . spooks me is there’s a delayed aspect of this,” Udall said. “If we’re looking at water that’s almost six years old, it means we may not see a problem until it’s six years too late, and that is particularly worrisome.”
Predicting streamflows
In Utah, Brooks said that accounting for groundwater storage has previously enabled his team to make more accurate predictions about runoff and flooding.
In 2021 and 2022, for example, Utah’s snowpack was around average, but Brooks said he and colleagues were able to correctly predict “horrendously low runoff” because they knew groundwater storage was meager. Similarly, in 2023, after a record snow year in Utah, the researchers accurately predicted only minor nuisance flooding because of the buffer of relatively low groundwater storage.
Water managers often look to April 1 snowpack readings as a barometer for the coming runoff season, but Brooks said gauging local groundwater storage in January or February can yield important insights about what’s to come in spring and summer.
“That simple number—how much groundwater is stored there relative to the long-term average—reduces your uncertainty in streamflow prediction by 50%,” Brooks said.
Measuring groundwater storage can be tricky, with nearby wells recording vastly different figures. “It’s really hard to get meaningful information on storage,” Brooks said.
However, the underlying geology across the West is widely understood from mapping and can be used to approximate whether an area has the potential to store more or less groundwater.
The researchers also argue that water managers can get a better handle on streamflows by employing tritium sampling themselves during the winter and the snowmelt season.
Brooks said he hopes the findings can be incorporated into hydrologic modeling and runoff predictions, but significant cutbacks in the federal workforce mean “it’s a very challenging time for water resource managers.”
“In an ideal world, we would have a National Science Foundation or NOAA or a private foundation say, ‘Hey, this could be really important. If we can predict streamflow more efficiently and earlier, we can make better decisions about how to manage it.’”
DWRe Water Talks: Help Shape the 2026 State Water Plan
Utah is updating its state water plan — a roadmap for how we manage, use and protect water across the state. The Division of Water Resources wants to hear from you! Water Talks are open, community gatherings where your voice helps shape the future of water in your region and beyond.
Attend your basin’s Water Talk, and:
- Learn about the plan and why it matters
- Tell us your community’s water priorities
- Share local knowledge and ideas
- Guide decisions for a resilient water future
Water Talks are coming to basins across the state this August, September and October. Meetings are still being scheduled, so if you don’t see your basin yet, check back!
Click here to view the Water Talk schedule.
What’s Next Analysis of the US Magnesium Superfund Site
US Magnesium has been making headlines again, and not in a good way. The facility has been non-operational since 2022, and on September 10, 2025, the company filed for Chapter 11 bankruptcy protection in Delaware, primarily due to environmental and financial liabilities.
So what does US Mag declaring bankruptcy mean for the Lake?
Since 2013, FRIENDS has overseen cleanup of the US Magnesium Superfund site through a Technical Assistance Grant from EPA. The purpose of the TAG is to inform members of the public of the threats posed by the Superfund site as well as what needs to be done to address those threats.
With that charge, we asked our Technical Assistant Grant advisor, Dr. Bill Johnson, to summarize the most immediate threats existing today, as well as his recommendations regarding the actions needed to understand the extent of those threats.
Read the summary report below:
What’s Next Analysis of the US Magnesium Superfund Site
By William P. Johnson, TAG Advisor to FRIENDS of Great Salt Lake
Published on 09-18-2025
Key Points:
- Though US Magnesium has not been operational since 2022, contaminants from the site still pose a significant threat to human and ecosystem health.
- Dr. Bill Johnson's risk assessments demonstrated that human carcinogenic risk and risks to birds, mammals, and benthic invertebrates drastically exceeded regulatory benchmarks.
- There is evidence that groundwater pathways have transported and continue to transport contaminants away from the site.
- The need to further understand the area’s hydrology and the extent of contamination is urgent, especially because the containment wall around the waste pond has not been completed.
- Recommended action steps:
- Review of existing documents to assess contamination in CERCLA portion as existed during operation.
- Resampling of well hydraulic heads and retrieval of samples for analysis of selected contaminants in wells to understand changes in groundwater flow and current contaminant concentrations.
- Installation of additional piezometers for sampling of hydraulic head and contaminants at five to ten sparsely monitored zones on perimeter of older Waste Pond and beyond.
- 3D modeling of contaminant transport conditioned to current hydraulic head and contaminant concentrations to simulate likely transport scenarios for contaminants outward from the site.
Additional background about US Magnesium and our TAG work.
10/9: Join us for our annual Fall Fundraiser!
Support our mission and programs, and enjoy dinner, drinks, music, and prizes with fellow friends of the Lake.
When: 6-9pm, Thursday, October 9, 2025
Where: The Garden Place at This Is The Place Heritage Park
The Office of the Great Salt Lake Commissioner (GSLCO), the Great Salt Lake Watershed Enhancement Trust (the Trust), and the Utah Division of Forestry, Fire and State Lands (FFSL) are excited to announce that up to $53 million in grant funding is available for projects that support Great Salt Lake and its wetlands.
The significant opportunity comes from three separate funding pools provided by each organization.
The Office of the Great Salt Lake Commissioner, in coordination with the Department of Natural Resources and the U.S. Bureau of Reclamation, is providing $50 million. This funding is available for projects in three categories:
- Voluntary Water Transactions: Compensation for a temporary or multiyear voluntary reduction in diversion of water or consumptive water use.
- System Conservation Projects: Voluntary system conservation projects that reduce consumptive water losses through improvements to water distribution infrastructure. *These System Conservation projects are distinct from the Upper Colorado River Commission's System Conservation Pilot Program.
- Ecosystem and Habitat Projects: Ecosystem and habitat restoration projects to address issues directly caused by drought in a river basin or inland water body.
“The abundance and diversity of funding available through this multi-organizational partnership will give project managers the opportunity to apply for the funding that will best fit their needs,” said Brian Steed, Great Salt Lake Commissioner. “The Commissioner’s Office is excited to coordinate its release of federal funding available for projects that will benefit the Great Salt Lake with our partners at the Great Salt Lake Watershed Enhancement Trust and the Utah Division of Forestry, Fire and State Lands.”
Of the remaining funding – $2 million will come from FFSL and $1 million from the Trust. FFSL and the Trust have partnered to provide up to $3 million for projects that will protect and/or restore wetlands and habitats in the Great Salt Lake’s surrounding ecosystems, benefiting the hydrology of the lake.
“This coordinated funding effort presents unprecedented opportunities to support projects that will enhance Great Salt Lake’s habitats, reduce water consumption, and deliver water to the lake and wetlands benefiting the overall health of Great Salt Lake. Partnership and collective efforts are essential to preserving Great Salt Lake,” said Marcelle Shoop, Executive Director of the Great Salt Lake Watershed Enhancement Trust. “The Great Salt Lake Watershed Enhancement Trust is honored to join the Utah Division of Forestry, Fire and State Lands and the Office of the Great Salt Lake Commissioner in announcing these new funding sources.”
This is the Trust’s third consecutive round of funding and second year partnering with FFSL. Drawing on past experience, the release of this funding has been timed to best align with the schedules of wetland managers.
“The Utah Division of Forestry, Fire and State Lands is excited to once again partner with the Great Salt Lake Watershed Enhancement Trust to release another round of wetland grant funding. The coordination of this funding opportunity with that of the Office of the Great Salt Lake Commissioner will support projects that will provide long-lasting benefits for Great Salt Lake’s hydrology and its associated wetlands,” said Ben Stireman, Deputy Director, Utah Division of Forestry, Fire and State Lands.
GSLCO, the Trust, and FFSL strategically coordinated the co-release of these funding opportunities to ensure a diverse range of project applicants and maximum impact for Great Salt Lake and its wetlands. Interested applicants will have the opportunity to assess which funding opportunity is right for their projects and apply through the corresponding grant process.
The Trust is managed by National Audubon Society’s Saline Lakes Program and The Nature Conservancy. The Trust and FFSL will work together on the coordination of the grant application and awarding process. This funding opportunity has specific project eligibility requirements and scoring criteria.
- https://www.gslwatertrust.org/wetland-grants
- https://greatsaltlake.utah.gov/great-salt-lake-water-delivery-program
Regardless of what grant process applicants select, all applications must be submitted by 11:59 p.m. MST on Oct. 24, 2025.
Office of the Great Salt Lake Commissioner -https://greatsaltlake.utah.gov/commissioner
Utah Division of Forestry, Fire and State Lands -https://ffsl.utah.gov/
Great Salt Lake Watershed Enhancement Trust -https://www.gslwatertrust.org/
9/5: Join us for our 12th annual Alfred Lambourne Arts Program gallery opening and reception
Join FRIENDS of Great Salt Lake for the Alfred Lambourne Arts Program gallery opening, readings, and performances on Friday, September 5 from 6:00-8:30 PM at Sorenson Community Campus (1383 South 900 West, SLC).
We'll experience artistic representations of our Inland Sea in the categories of visual arts, literary arts, movement, and sound. Free and open to the public.
Click here to read more about the program and view catalogs of past submissions.
Click here to watch our short video about the program, Protection in Perpetuity: Arts
The State Engineer will be holding a public meeting to discuss the Great Salt Lake Distribution Management Plan on July 31, 2025, at 6 pm in the DNR auditorium in Salt Lake City. There will also be a virtual option for those who can’t attend in person. The relevant information and details can be found on our website: https://www.waterrights.utah.gov/GSLDMP
This plan will determine how the State Engineer regulates the distribution, measurement, and appropriation of water within the GSL meander line.
Meeting Details:
Who: Water Users, Stakeholders, and Interested Parties
When: July 31, 2025, 6:00 p.m. to 7:00 p.m.
Where: Department of Natural Resources Office, DNR Auditorium (Room 1040), 1594 W North Temple St, Salt Lake City, Utah 84116
Online: Remote participation and live stream broadcast of the public meeting will also be available on-line at the following links:
- Google Meeting: https://meet.google.com/wqa-dydu-agh
- Live Stream: https://youtube.com/live/G2h26aYv92w
Purpose: In accordance with Chapter 73-33, Utah Code Annotated, the State Engineer has been directed to "regulate the measurement, appropriation, apportionment, and distribution of water within the Great Salt Lake meander line" through the creation and adoption of a Distribution Management Plan for the Great Salt Lake
The public is invited to attend a public meeting wherein a draft of the Great Salt Lake Distribution Management Plan will be presented and the public will have an opportunity to ask questions or provide comments.
Agenda:
- Introduction: Teresa Wilhelmsen, P.E. - State Engineer
- Great Salt Lake Distribution Management Plan presentation: Blake Bingham, P.E. - Deputy State Engineer
- Public Comments & Questions
Materials: Printable Agenda
Public Comments:
Please submit written comments to:
Mailing Address: Utah Division of Water Rights, 1594 West North Temple Suite 220, PO Box 146300, Salt Lake City, UT 84114-6300
Email: waterrights@utah.gov
Subject: Public Comment Regarding GSL DMP
State regulators allowed the mining company to edit and soften investigations into its potential role in rising impacts to shorebirds.
By Leia Larsen | July 14, 2025, 6:00 a.m. | Updated: 9:12 a.m.
Click here to read this article on the Salt Lake Tribune website.
A polluted tailings pond at the Kennecott Utah mine has sunk 20 feet or more, raising concerns that it has seeped contamination into the neighboring Great Salt Lake.
But the state regulator charged with protecting the lake’s water, the Division of Water Quality, allowed the mining company to edit this information out of a recent groundwater permit, documents show. The division also allowed Kennecott to quietly nix a study that would have investigated the tailing pond’s connectivity to and impact on the Great Salt Lake.
“Why? Because it would cost more to dig deeper?” said Lynn DeFreitas, executive director of FRIENDS of Great Salt Lake. “It undermines a level of confidence in the process and assurance that things are on the right track to keep the system healthy and keep industry accountable.”
Division director John Mackey said it’s common to let regulated companies weigh in on their permits.
“It’s necessary for us to coordinate with them on that,” Mackey said, “because they’re the ones who need to do the study.”
Kennecott denied settlement of its tailings pond posed an environmental concern in an emailed statement from the mine’s parent company, Rio Tinto.
“We stand by the safety of our tailings impoundment,” a spokesperson wrote, “and affirm that it meets, and in some cases exceeds, the required standards.”
At issue is selenium, a mineral that can be toxic for humans at high concentrations. It also poses a threat to the millions of migrating birds that visit the Great Salt Lake every year. The material weakens eggs and deforms embryos. It can bioaccumulate in the wetland bugs those birds eat, and work its way up to hunters harvesting waterfowl.
More than a century of mining at the Kennecott facility has resulted in two groundwater plumes with selenium pollution that have impacted at least some the lake’s wetlands.
After the U.S. Environmental Protection Agency listed Kennecott’s North Zone near the Great Salt Lake as a superfund site in the 1990s, the mining company reached an agreement with Jordan Valley Water District to build a reverse osmosis plant to remove selenium and treat groundwater to drinking standards.
Both Kennecott and Jordan Valley have permits with the state to discharge selenium-ladened wastewater to Gilbert Bay, the main body of the Great Salt Lake, but they adjust their releases throughout the year to not interfere with nesting season.
Last year, however, regulators detected elevated levels of selenium in migratory bird eggs collected near the discharge point.
Given the rise of selenium in eggs, and Kennecott’s “inability” to manage selenium-impacted groundwater, the EPA determined in September that the wetlands and groundwater near the tailings pond are not protective of human health and the environment in a five-year review.
The agency’s report noted the Utah Division of Water Quality had alerted the federal agency about a 20-foot settlement of the tailings pond, which has a natural clay liner meant to prevent pollution from mingling with the shallow aquifer.
But deterioration of the pond that is causing it to sink at a rate of 0.9 feet a year could mean there’s a pathway for tailings water to mix with groundwater and the surrounding wetlands, EPA noted.
When the Utah Division of Water Quality finalized its own review of Kennecott’s tailings pond for a renewed groundwater discharge permit in June of 2024, it required Kennecott to conduct “investigations” of the problem, according to records obtained by FRIENDS of Great Salt Lake and shared with The Salt Lake Tribune.
The agency gave the mining company one year to analyze how the pond’s sinking has impacted its clay liner and whether it was seeping.
It also gave Kennecott two years to file a “Great Salt Lake Connectivity and Influence Analysis,” studying water quality and how the lake’s briny water mixes with surrounding fresh water and groundwater.
A division scientist shared the draft permit language with Rio Tinto on June 5, 2024, emails show.
A staffer for the mining company responded three weeks later, attaching a document with significant red lining and revisions.
Gone were the references to the tailing pond’s settlement.
Gone was the requirement to analyze the pond’s seepage.
Gone was the requirement to study connectivity to the Great Salt Lake.
The mining company also replaced the word “investigations” with “evaluations.”
The Division of Water Quality submitted the revised permit for public comment on July 25 last year. It approved the permit in December, which is up for renewal again in 2029.
The revised language requires Kennecott to compile “previously submitted reports” on the pond’s clay layer and hydrology — suggestions made by the company in its revisions. The Division did, however, insert that its director “may” later require an investigation.
That same month it approved the groundwater permit, the division received a report that detected selenium in five shorebird eggs at levels that trigger a regulatory response. It’s the first time the state has detected such high concentrations of the contaminant, Mackey confirmed, although the state has not reported egg selenium data since 2017.
That’s because receding lake levels made egg collection difficult, he said.
Kennecott also reported pond settlement to the Division of Water Rights, which oversees dams in the state, in 2022. Regulators have known about the issue for a “long, long time,” Mackey said.
“The sheer weight of of the embankments themselves, which are huge,” the division director said, “are designed with settlement in mind.”
But the pond’s sinking came as a surprise to environmental watchdogs like FRIENDS of Great Salt Lake.
“The assurance that the public had from Kennecott about the tailings impoundment was ... ‘it’s fine,’” DeFreitas said, “‘it’s protected, it’s not going to go anywhere.’”
Mackey further noted that his division did add a requirement to Kennecott’s renewed permit for an evaluation of the brine mixing zone between the Great Salt Lake and the freshwater aquifer below the facility.
Issues with elevated selenium will be evaluated through surface water permits, he said.
“Not to say that we’re not concerned or not interested in the possibility that the selenium might be coming from [the] tailings,” Mackey said.
In December, after receiving reports on the shorebird eggs, Mackey issued letters to both Kennecott and Jordan Valley Water Conservancy District, notifying both to prepare to take actions reducing selenium unless they could prove they were not the source of increased pollution.
The water district declined to comment, but Mackey’s letter notes it had reduced selenium discharge during the nesting season.
Kennecott, meanwhile, has not released surface water to the Great Salt Lake for more than a year, Mackey said. The mining company mostly recycles water from its ponds.
In both cases, Mackey wrote he did not have sufficient evidence to determine whether the entities were in the clear.
FRIENDS, meanwhile, has opted to fund its own study of possible groundwater contamination pathways to the Great Salt Lake, and whether the dropping tailings pond has played a role in climbing selenium.
The U.S. Geological Survey will lead the lake-wide study, with help from Brigham Young University and the University of Utah. Fieldwork is expected to begin this month.
This article published through the Great Salt Lake Collaborative, a solutions journalism initiative that partners news, education and media organizations to help inform people about the plight of the Great Salt Lake. Read all of our stories atgreatsaltlakenews.org.
Click here to view the full draft.
In 2024 House Bill 453, the Utah State Legislature tasked the Utah Division of Water Quality (DWQ) with developing rules that establish a salinity limit for discharges to Great Salt Lake (GSL) that occur as part of the mineral or element extraction process. The relevant bill text was codified to Utah Code Section 73-33-203.
Section 73-33-203 Relevant Text
(a) On or before June 1, 2025, the Division of Water Quality, in consultation with the Division ofForestry, Fire, and State Lands, and in cooperation with the Great Salt Lake commissioner pursuant to Section 73-32-203, shall make a rule, in accordance with Title 63G, Chapter 3, Utah Administrative Rulemaking Act, setting a limit for the salinity of water or brine that a person may discharge into the Great Salt Lake as part of the mineral or element extraction process.
(b) If a person discharges water or brine that exceeds the limit imposed under Subsection (3)(a),the Division of Water Quality may modify, revoke and reissue, or terminate any permit issued by the Division of Water Quality related to the discharge.
DWQ’s goal for this rule is to establish limits for the salinity of water or brine that a person may discharge into GSL as part of the mineral or element extraction process that prevent potential negative effects of high salinity discharges on GSL chemistry or biota, particularly when the salinity in Gilbert Bay of GSL is at or approaching a critical level.
Stay Tuned: 2026 Great Salt Lake Issues Forum
