By John Fleck, Anne Castle, Eric Kuhn, Jack Schmidt, Kathryn Sorensen, and Katherine Tara
As we await Friday’s (Aug. 15, 2025) release of the Bureau of Reclamation’s Colorado River 24-Month Study, we need to remember a painful lesson of the last five years of crisis management: whatever you see in Reclamation’s report of the “Most Probable” reservoir levels for the next two years, we must prepare for things to be much worse.
A year ago, Reclamation’s “Most Probable” forecast told us to expect Lake Powell to hold 10.36 million acre feet of water at the end of July 2025, with a surface elevation 3,593 feet above sea level. Actual storage in Powell at the end of July was 7.46 maf, 2.9 million acre feet less, and the reservoir is 38 feet lower, than the “Most Probable” forecast.
Four years ago, one of us (Eric Kuhn) wrote this, which is helpful in understanding what is happening:
The problem: the assumptions underlying the study do not fully capture the climate-change driven aridification of the Colorado River Basin.
In 2022, a Utah State Center for Colorado River Studies team led by Jian Wang (including one of us, Schmidt) took this on in more technical detail – Evaluating the Accuracy of Reclamation’s 24-Month Study of Lake Powell Projections. The finding provided technical support for an intuition water managers already had: the 24-Month Study has an optimistic bias.
It is a practical demonstration of the problem U.S. Geological Survey scientist Paul Milly and colleagues famously warned us about nearly two decades ago – in water management, climate change means the past is increasingly unhelpful in projecting the future.
The 24-Month Study: A Brief Primer
Produced monthly, Reclamation’s 24-Month Study includes three scenarios: Most Probable, Minimum Probable, and Maximum Probable. The Study includes 18 pages of data and forecasts for twelve Colorado River system reservoirs, from Fontenelle and Flaming Gorge in the north to Mohave and Havasu in the south, projecting things like elevation, storage, inflows, releases, evaporation, and hydropower production each month for the next two years.
Here is Wang et al’s explanation of how it works:
Projections for reservoir elevations during the next few months are based on predictions of reservoir inflow using a widely accepted watershed hydrologic model run by the Colorado Basin River Forecast Center. The input data for that model are observed snowpack in the watershed, soil moisture, and anticipated precipitation and temperature. Projections for reservoir elevations beyond the immediately proximate winter, a year or more in the future (‘second year projections’), are based on statistical probabilities calculated using analyses of past inflows during a 30-year reference period.
The resulting model runs represent a wide range of uncertainties, which are captured in three resulting scenarios:
- Most Probable: the middle of the range
- Maximum Probable: the 90th percentile scenario, meaning that 10% of the model runs predict even wetter hydrology and 90% predict drier.
- Minimum Probable: the 10th percentile scenario, meaning that 10% of the model runs predict even drier hydrology and 90% predict wetter.
The problem, implicit in the argument Milly et al. made nearly two decades ago, is that a 30-year reference period is no longer a reliable indicator of what we should expect in the future. It represents a river we no longer have. This is not to suggest any bias or partiality on the part of Reclamation, but merely that the algorithms and modeling used to produce the 24-Month Study have proven in recent years to be skewed more toward the the past than the true-to-life. Our response needs to reflect that reality.
Because of the changing conditions in the Colorado River Basin, the Minimum Probable scenario has become the most valuable in providing a reliable indicator of the future. Actual flows and reservoir levels have been tracking the minimum probable forecast since March of this year. As we enter the fall of 2025, with the weak summer monsoon for most of the Upper Basin coupled with weak La Niña conditions persisting through the fall and early winter, and NOAA’s seasonal outlook pointing to a warmer and drier than average fall, it’s a good bet that this trend will continue at least through mid-winter. The Basin should be prepared for minimum probable conditions, with a clear possibility that actual conditions could be worse than the 10th percentile scenario. The basin community needs to be ready to respond with the necessary water use reductions now to protect the Colorado River system on which we all depend.
Sources:
- August 2024 24-month studies: Most probable, Minimum probable
- July 31, 2025 reservoir levels: USBR hydro data
We will continue to allocate the maximum water we think we might get. We certainly shouldn’t presume that people will leave any significant buffer.
DJT will just declare the records of discharge biased and invalid Or, his Secretary of the Interior, who as I understand it can single-handedly dictate apportionment under severe drought conditions, will distribute water by extortion. 2490, here we come.
Meanwhile Phoenix ads 15,000 more housing units.
Here are the updated projections:
https://www.usbr.gov/lc/region/g4000/riverops/24ms-projections.html
The bottom line on this is we can’t predict the weather no more than ten days in the future. Everything longer than that is based on a model and models have uncertainty and tend to be unpredictable when new variables add the equation.
The other takeaway from this article was Reclamation’s long term models were optimistic. There is a double edged sword here. Had the models been more pessimistic, someone surely would have complained when restrictions were applied. The case I would mention here was when water was withheld from the farmers in the Klamath Irrigation District due to the Endangered Species Act ESA some years back. As I recall the local farmers echoed, “Our family has been here for generations and we will lose the family farm.” When you factor in that they voiced their concerns to the local Congressman and the State Senators, you can appreciate the pressure applied to both DoI and Reclamation in particular. Now apply that to a model that tends to be more restrictive and cuts need to be made. There will be complaints. On a basin level.
Reclamation bends over backwards to work with Stakeholders. Seen it firsthand.
Easy for someone from academia to point out flaws in someone’s policies, methods of managing the river or mathematical models. There is more to the equation. Factor in politics and commerce.
From my own perspective, commerce is the bottom line.
Don’t forget about San Juan River & diminishing flows due to pipeline diversions to Navajo Nation with more on the books at Chinle wash & Paiute Farms – water for Kayenta and Monument Valley. I mentioned the below info to Mike DeHoff RE: Returning Rapids Project.
I wasn’t sure how much Mike had researched these major diversion projects in NM to Navajo Nation. I’ve tried to keep up with all this over the years since the Buckman Diversion tunnel diverted water to Chama-Santa Fe-Albuquerque several decades ago.
As you all are probably aware, that old Colorado River Compact (1922) gave lip service to Indian water rights, but was never seriously considered until the last few decades. Even then, money allocations have been herky-jerky and progress slow.
https://www.usbr.gov/uc/water/crsp/cs/nvd.html
Diversions to Cutter Reservoir for the Navajo Indian Irrigation Project (NIIP) and the Navajo Gallup Water Supply Project (NGWSP) are 152 cfs. The San Juan-Chama project was diverting at a rate of 31 cfs.
https://nmpoliticalreport.com/2025/04/18/years-in-the-making-work-begins-on-navajo-gallup-water-treatment-plant/
just now getting around to water treatment plant for the San Juan Lateral (main line to Gallup/Window Rock)
BoR needs to look at these diversions along with climatic conditions in making their ‘guestimations’
“Climate models reveal how human activity may be locking the Southwest into permanent drought”
Pedro DiNezio, Associate Professor of Atmospheric and Ocean Sciences, University of Colorado Boulder and Timothy Shanahan, Associate Professor of Geological Science, The University of Texas at Austin (The Conversation, August 13, 2025 https://theconversation.com/climate-models-reveal-how-human-activity-may-be-locking-the-southwest-into-permanent-drought-262837)
“..The PDO was thought to be a natural phenomenon, governed by unpredictable natural ocean and atmosphere fluctuations. But new research published in the journal Nature suggests that’s no longer the case.
Working with hundreds of climate model simulations, our team of atmosphere, earth and ocean scientists found that the PDO is now being strongly influenced by human factors and has been since the 1950s. It should have oscillated to a wetter phase by now, but instead it has been stuck. Our results suggest that drought could become the new normal for the region unless human-driven warming is halted.”
Nature (13 August 2025) “Human emissions drive recent trends in North Pacific climate variations https://www.nature.com/articles/s41586-025-09368-2
Thank you for writing this and your recent email. IDK if you have noticed but our New Mexico media has ignored Reclamation’s dismal forecast of water levels in Lake Powell even though it has been reported in national media and in other Southwestern states. I sent a news tip the New Mexican, the ABJ Journal, Source NM, and Searchlight NM. It will be interesting to see which, if any, cover the story. I suggested you as a New Mexico based expert. I hope you don’t mind.
I have been driving a lot this year between Tucson and Durango. Each trip up the Mogollon Rim and across the Colorado Plateau and the Navajo Reservation I look for evidence of events that are moving water and sediment. Very very rare this year and it reminds me of what the landscape looked like in 2022 and in the early 2000’s as aridification came to the 4-Corners. As I cross the San Juan River at Shiprock I always look at the levels of the San Juan River and for most of the summer it has been running bank full. Without rain this means that the likely reason is they are moving water from Navajo Reservoir to Powell to prop up Lake Powell levels. Based on the recent (August 2025) BOR 24-Month Study I can only surmise even when viewed through the optimistic lenses of Reclamation forecasters that this winter and WY 2025 is going to be a real test of our water management skills. I hold little faith in the administration or Congress providing any leadership or forward thinking. I am very concerned that the Administration has cripppled the technical capacity of BOR to understand and manage the reality of the water and climate situation.
The September 2025 24-MS just released is a real doozy. Looks like Section 6.E of the 2024 Near-Term Colorado River Ops has been triggered looking at the minimum probably results. We are in for a bumpy ride in WY 2026…