LAKE POWELL
A river divided on paper
Seven states, one treaty, and an arithmetic error that every shortage on this page descends from.
On 24 November 1922 seven states signed the Colorado River Compact at Santa Fe. It apportioned 7.5 million acre feet a year to each basin, upper and lower, out of a river the negotiators took to be about 16.4 million .
They measured it during the wettest stretch on record THE ERROR AT THE CENTRE
The flow record they used came from an unusually wet period. The river has not delivered that average since. Every shortage, every emergency release and every lawsuit threat on this page descends from a division of water that was larger than the water .
The obligation that matters is Article III(d). The Upper Division states must not deplete the flow at Lee Ferry below 75 million acre feet in any ten consecutive years . Lake Powell is the only physical means they have of meeting that in a run of dry years without cutting off their own users.
Mexico is guaranteed 1.5 million acre feet a year under the 1944 treaty . Tribal water rights across the basin remain substantially unquantified, which means a large volume of legally senior water has not yet been added to the arithmetic.
Two agreements govern how Glen Canyon Dam is operated. The 2007 Interim Guidelines, signed 13 December 2007, coordinate Powell and Mead and set the Lower Basin shortage tiers . The 2019 Drought Contingency Plans sit on top of them. Both run out at the end of 2026 .
The states did not agree FEBRUARY 2026
The basin states missed a 14 February 2026 deadline to produce a consensus proposal. Interior said it would proceed with post 2026 guidelines without full state consensus .
Reclamation published a Final Environmental Impact Statement, with the Federal Register notice of availability on 31 July 2026. No Record of Decision had been published as of the end of that month , and new rules are required by 31 December.
Sixty three years of water
Filled once, held for twenty years, and falling since 2000.
Read the shape. The reservoir took seventeen years to fill, from 1963 to 1980. It sat near full pool through the 1980s and 1990s. It has been coming down since about 2000, and the last five years have taken it lower than anything since it was filling.
| Figure | Value | Source |
|---|---|---|
| Record high | 3,708.34 ft, mid July 1983 | Federal survey |
| Record low | 3,519.92 ft, April 2023 | Reclamation |
| Full pool | 3,700 ft, the top of the spillway | Reclamation |
| Full pool, newer datum | 3,702.91 ft above NAVD 88 | Federal survey |
| Minimum power pool | 3,490 ft | Reclamation |
| Dead pool | 3,370 ft | Reclamation |
| End of June 2026 | 3,525.17 ft, 5.60 million acre feet, 24 per cent of live capacity | Reclamation |
| Projected end of water year 2026 | about 3,516.18 ft, 22 per cent | Reclamation |
| Most recent published reading | 3,520.95 ft on 9 August 2026 | Park Service |
| When | Action |
|---|---|
| 3 May 2022 | 480,000 acre feet held back from release to Lake Mead, and about 500,000 acre feet released from Flaming Gorge, to defend elevation 3,490 |
| 2022 | Annual release cut from 7.48 to 7.0 million acre feet |
| 17 April 2026 | 660,000 to 1 million acre feet ordered from Flaming Gorge, and the Lake Powell release cut from 7.48 to 6.0 million acre feet, aiming to hold the reservoir at or above 3,500 feet by April 2027 |
| Water year 2026 | Operating in the Mid Elevation Release Tier at 6.0 million acre feet |
June 2026 inflow was 12 per cent of average THE UNDERLYING PROBLEM
Unregulated inflow to Lake Powell in June 2026 was 306 thousand acre feet, which Reclamation puts at 12 per cent of average. The release that month was 508 thousand acre feet . More than half again as much left as arrived.
The water year 2026 inflow forecast was 3.50 million acre feet, 36 per cent of average .
What the dam can and cannot do
Three elevations, four pipes, and an inspection report the agency has described three different ways.
Reclamation states it plainly: "Water cannot be released from the penstocks below a Lake Powell elevation of 3,490 feet (which is known as the minimum power pool and is 20 feet above the penstock intake centerline)."
Below that, everything goes through the river outlet works: four 96 inch pipes with a combined capacity of 15,000 cubic feet per second, against 33,200 through the penstocks and 276,000 through the spillways at full pool .
Reclamation says its own engineers do not want to rely on them 2024 DECISION MEMO
After an inspection following a 2023 high flow experiment, Reclamation's Technical Service Center wrote that "it is recommended that Reclamation not rely on the river outlet works as the sole means for releasing water from Glen Canyon Dam as would be required at reservoir levels below the minimum power pool, elevation 3,490 ft."
The same memo imposed reduced interim maximum flows through each conduit, tightening as the reservoir falls. At 3,490 feet the design maximum is 3,680 cubic feet per second per conduit and the interim limit is 3,185 .
Three Reclamation documents, three different tones A CONFLICT WORTH PRINTING
The same 2023 finding is described three ways by the same agency, and this page prints all three rather than picking the reassuring one or the alarming one.
The 2024 news release: "new and minor cavitation damage", and "The dam's infrastructure and its ability to deliver water downstream are not at risk" .
The 2024 technical decision memorandum: do "not rely on the river outlet works as the sole means for releasing water" below 3,490 feet .
The post 2026 technical appendix: the 2023 inspection found that for continued long term operation, "major repairs or replacement of the hollow-jet valves should be considered" .
Glen Canyon Powerplant has eight generators and a combined capacity of 1,320 megawatts at full pool . At minimum power pool that falls to an estimated 630 megawatts . Below it, zero.
Generation has fallen from about 4,600 gigawatt hours a year before 2000 to 2,590 in 2022 .
The spillway capacity is published two ways as well 208,000 OR 276,000
Reclamation's Dams database says 208,000 cubic feet per second at elevation 3,700. Reclamation's own 2024 decision memo and the federal dam register both say 276,000 . This page is not going to average them.
Sand, air, and fish
The reservoir is filling with sediment, leaking into sandstone at a rate nobody has measured since the 1970s, and letting predators through the dam.
The federal survey resurveyed Lake Powell with multibeam sonar and lidar in 2017 and 2018, the first full resurvey since 1986 . What it found:
| Total storage capacity, 2018 | 25,160,000 acre feet |
|---|---|
| Lost since 1963 | 1,833,000 acre feet, or 6.79 per cent |
| Lost since 1986 | 1,048,000 acre feet, or 4 per cent |
| Average annual loss | about 33,270 acre feet a year |
The sediment is not spread evenly. It arrives where the rivers arrive, which is why the north end of the lake silted up first and why Hite stopped being usable long before the rest of it.
This is the most argued over set of numbers on the Colorado, and the honest summary is that the measurements are old.
What the assessment found
Utah State University's Center for Colorado River Studies assessed the Fill Mead First proposal in 2016 . Lake Mead evaporation was measured at 6.0 feet a year between 2010 and 2015. Lake Powell evaporation is carried at 5.7 feet a year, from measurements taken between 1965 and 1979.
"It cannot be demonstrated that total evaporation would be significantly different from the estimated losses under the equalization rule."
On the bottom line: "The projected water savings by implementing FMF would be less than ~0.05 million af/yr (~50,000 af/yr)."
The authors' own recommendation is not for or against the proposal. It is for a measurement programme: modern evaporation measurement at Powell, continued measurement at Mead, new observation wells, and gauging of ungauged tributaries .
What the two camps do with it
Advocates of draining put the figure far higher. Glen Canyon Institute publishes "Up to 300,000 acre feet of water could be saved each year", attributing it to a 2013 paper, and says the reservoir loses about 860,000 acre feet a year to evaporation and bank seepage combined .
Arizona's water agency answered directly in 2017: "Seepage losses into the Lake Powell bedrock, meanwhile, likely are ten percent of what Fill Mead First advocates claim." and "The magnitude of potential ecosystem changes caused by the FMF plan are so great and the water savings are so uncertain that implementation should await a new program."
Both sides quote the same USU paper accurately. Opponents quote the 50,000 acre feet figure and stop. Supporters quote the uncertainty language and the call for new measurement. The paper does not say the proposal is a bad idea. It says the water saving case for it is unproven on forty year old data.
This is the live ecological story, and it runs the opposite way to what most people expect. The problem is not what is happening in the lake. It is what is getting out of it.
Smallmouth bass below Glen Canyon Dam AN IMMINENT THREAT, SAYS THE AGENCY
The Park Service: "Lower lake levels at Lake Powell and rising temperatures in the Colorado River have increased concerns that non-native, warm-water predators...could pass through the dam..." and "If these warm-water predators establish populations below the dam, it is likely they would negatively impact native fish communities in Glen Canyon National Recreation Area and in Grand Canyon National Park."
Reclamation on the mechanism: "Invasive fish, such as smallmouth bass, residing in the upper layer of Lake Powell, are now more likely to pass through the dam and spawn downstream." and "If left unchecked, their expanding population could pose a major threat to federally protected fish species such as the humpback chub."
A regional director called it "an imminent threat to the recovery of the humpback chub" .
There is a statute behind this. The Grand Canyon Protection Act of 1992 requires the Secretary to operate Glen Canyon Dam so as to protect and improve the values for which Grand Canyon National Park and Glen Canyon National Recreation Area were established . Protecting the chub is not a preference. It is a duty attached to how this dam is run.
The tool built for this is called a cool mix release. Part of the dam's daily discharge is sent through the deeper outlet works, which draw colder water from lower in the reservoir, to hold the river below the dam under the temperature bass need to spawn. The trigger is "three consecutive days of average river temperatures exceeding 60°F at river mile 30 below the dam" . The federal survey reported that since it was introduced "there has been no evidence of smallmouth bass reproduction in this stretch" .
It was not run in 2026 AUGUST 2026
Reclamation confirmed in early August 2026 that it would not release cool water this year, citing "significant hydropower generation concerns under already strained system conditions", and said its efforts would focus on rapid response removal, backwater modification and structural solutions instead .
That is a letter, not a number, on purpose. Those are agency statements, but we could only retrieve them through news coverage. We could not find a Reclamation release announcing the decision. If you need this for anything that matters, ask Reclamation public affairs directly.
The gauges
Where the water arrives, where it leaves, and where the measurements are taken.
A reservoir holding a quarter of its capacity, thirty one feet above the point where the power stops.
Fathom Restoration is a Utah 501(c)(3) that runs volunteer dive cleanups and is built to pull what does not belong out of Utah water, statewide.
Donate to Protect Lake PowellEvery figure on this page is quoted from the source named beside it.
Sources
How these were checked
Every figure is quoted from the sentence or the database field it appears in. Numbered sources are agencies, courts and Congress. Lettered sources are everything else, including advocacy organisations, academic work outside an agency, and journalism, and they are never used for a number an agency also publishes. Where sources disagree, all of them are printed. On this page that happens over the spillway capacity, the powerplant nameplate, the full pool datum, evaporation and seepage rates, and the severity of the 2023 damage to the river outlet works, which the same agency describes three different ways in three different documents.Colorado River Compact, 1922
Glen Canyon Dam and Lake Powell, report R47497
Management of the Colorado River, report R45546
Colorado River post 2026 operations
Colorado River interim guidelines, 2007
News release 5326, 17 April 2026
News release 5283, 14 February 2026
News release 4196, 3 May 2022, drought response actions
Glen Canyon Dam, water operations, current status
Interim operating guidance for Glen Canyon Dam at low reservoir levels, technical decision memorandum, 2024
News release 4946, 3 September 2024, river outlet works relining
Post 2026 environmental impact statement, technical appendix 15, dams and electrical power
Elevation area capacity relationships of Lake Powell in 2018, Scientific Investigations Report 2022 to 5017
Lake Powell's storage capacity updated for the first time since 1986, 21 March 2022
Lake Powell water year maximum and minimum time series
Lake Powell at Glen Canyon Dam, gauge 09379900
Colorado River at Lees Ferry, gauge 09380000
Glen Canyon Dam, facts and figures
National Inventory of Dams, record AZ10307 Glen Canyon Dam
Changing lake levels
Fill Mead First, a technical assessment, white paper 1, 2016
Fill Mead First plan to drain Lake Powell has sprung some big leaks, 22 February 2017
News release 5205, 5 August 2025, cool water releases
Smallmouth bass below Glen Canyon Dam
Putting fish on ice, designer flows to stop an invasion in Grand Canyon, 24 February 2025
Slough channelization below Glen Canyon Dam, 5 June 2025
Boating in Utah in 2026, quagga mussel requirements, 15 April 2026
Boating at Lake Powell, aquatic invasive species procedure
Upper Colorado hydrologic data, Lake Powell site 919
Grand Canyon Protection Act, 1992
National Hydrography Dataset, Lake Powell water body, COMID 3528295
Fill Mead First
Colorado River basin storage continues slide toward system crash, 1 June 2026
No Lake Powell cool water releases this year, 8 August 2026
Colorado River developments and potential compact litigation
As Lake Powell recedes, scientists document the return of life to Glen Canyon, 1 July 2026
What these numbers are measuring
The two flow cards are the whole story of a reservoir. One gauge sits on the Colorado main stem near Cisco, well above the lake. One sits below the dam at Lees Ferry . When the second is larger than the first, the reservoir is being spent.
Neither of them is a complete inflow or outflow figure. Cisco is above the confluences with the Green and the San Juan, and it is a long way above the lake. Lees Ferry is the real release, but it is fifteen miles downstream of the dam. Reclamation publishes proper monthly inflow and release volumes and those are the numbers to use for anything that matters.
Below full pool is arithmetic. It is 3,700 feet minus the gauge reading . The federal survey states full pool as 3,702.91 feet on a newer vertical datum , so the two agencies are about three feet apart before anyone measures anything.
Everything from these gauges is provisional until the survey reviews it, and the cards say so when the data does. Reclamation separately publishes its own daily record for this reservoir back to 28 December 1963 .