Two weeks after scientists announced that the humpback chub had staged a historic comeback in the western Grand Canyon, the Bureau of Reclamation canceled the cold-water releases that protect the fish from an invasive predator. The timing captures, in miniature, the dilemma now squeezing the entire Colorado River system: a river so depleted that the tools designed to save it have become too expensive to run.
On August 6, Upper Colorado Basin Regional Director Wayne Pullan informed partners by email that Glen Canyon Dam would not release its annual cool mix of deep, cold Lake Powell water in 2026, citing significant hydropower generation concerns under already strained system conditions. The announcement came twelve days after a new federal study estimated that adult humpback chub in the western Grand Canyon had grown roughly 160-fold between 2010 and 2024, to a median of about 70,000 fish.
A rebound, and the threat that shadows it
The humpback chub, a silvery fish with a fleshy hump behind its head, was listed as endangered in 1967 and became one of the Grand Canyon’s most fragile native species after Glen Canyon Dam trapped the river’s sediment and chilled its waters. By the early 2000s, the largest aggregation, at the Little Colorado River confluence, had dwindled to a few thousand fish. In 2021, the species was downlisted from endangered to threatened.
The new study, published in the Canadian Journal of Fisheries and Aquatic Sciences, found that chub in the western Grand Canyon have thrived in recent years, helped by warmer water, scarce predators, and the partial barrier that Pearce Ferry Rapid poses to non-native fish moving upstream. But the paper’s authors were unambiguous about the caveat: invasive fish remain the central concern, and where smallmouth bass establish themselves, chub become prey.
Smallmouth bass were stocked in Lake Powell for sport fishing in 1982 and thrived in the warm reservoir. In 2022, biologists found the first evidence of them reproducing below the dam. The bass pass through the dam’s turbines from the lake’s warm surface layer, and roughly half survive the passage, according to a recent study cited by the Associated Press. In the river below the dam, the two species occupy the same thermal niche: chub spawn at water temperatures of 16.1°C (61°F) or warmer, and bass begin reproducing at about 15.5-16°C (60-61°F).
The cool mix was designed to break that overlap. Cold water drawn from deep in the reservoir, through bypass tubes that never spin a turbine, cools the river below the spawning threshold for bass. Reclamation’s standing trigger is an average river temperature above 15.5°C (60°F) for three consecutive days at river mile 30. This summer, the river ran near 20.5°C (69°F), and the threshold was exceeded regularly for more than a month. No release came.
The tool had worked. After cool-mix releases in 2024 and 2025, no young bass were found below the dam, and combined with mechanical removal by the National Park Service, bass numbers declined where they had taken hold. Fish biologist Kevin Bestgen of Colorado State University called the releases very effective.
The cost of cold water
The price of that success is measured in megawatts. Water sent through the bypass tubes generates no electricity, and the Western Area Power Administration had to buy replacement power on the open market during the 2024 and 2025 programs. The Colorado River Energy Distributors Association, which represents about 155 hydropower customers, argued in a May letter to Interior Secretary Doug Burgum that the releases could cost more than $20-30 million per year and are not a sustainable fix. Its executive director, Andy Colosimo, said the cancellation was the right call given the conditions.
Conservation scientists disagree with the calculus. Heather Whitlaw, field supervisor for the U.S. Fish and Wildlife Service, said skipping the releases amounts to giving up on the future of humpback chub recovery. Matt Rice, southwest regional director for American Rivers, called the decision disappointing but not unexpected, and said he hoped it would not be a death sentence for the chub. The warm water also threatens the reach’s rainbow trout fishery, which lost nearly half its fish to a heat event in 2022.
The spring high-flow experiment, a separate management tool that releases large pulses of water to rebuild sandbars, was also skipped this year. Reclamation’s official explanation cites insufficient consensus among the interagency planning team; the team is expected to reconsider a flood for fall 2026. Biologist David Lytle of Oregon State University warned that skipping floods for multiple years produces a slow decline in the quality of the ecosystem.
A system at its limits
The backdrop is a river in freefall. Lake Mead, the system’s largest reservoir, fell to a record low of 317.1 meters (1,040.4 feet) on August 7, the lowest since it was filling in 1937, and now holds about 27 percent of capacity. Hoover Dam stops generating at 289.6 meters (950 feet) and reaches dead pool at 272.8 meters (895 feet). Lake Powell, impounded by Glen Canyon Dam, sits near 24 percent of live capacity, with hydropower generation at risk below 1,064 meters (3,490 feet).
The Colorado River supplies more than 40 million people across seven states, tribal nations, and Mexico. The 1922 compact that divides its waters expires at the end of 2026, the states failed to agree on a replacement, and Reclamation said it will impose its own management strategy starting in October, with authority to cut releases significantly. The emergency measures announced in April, including cutting Powell’s release from 7.48 to 6.0 million acre-feet, may push the upper basin below its compact delivery obligations, raising the prospect of the first interstate litigation over the river.
What happens next
With the cool mix shelved, Reclamation says it will rely on expanded rapid-response fish removal, backwater modifications, and the advancement of durable structural solutions, a category that could eventually include selective withdrawal towers capable of pulling cold water from depth without sacrificing generation. Whether bass reproduced below the dam this summer is not yet publicly reported; fall monitoring will provide the first answer.
There is also a quieter risk. Budget cuts to the USGS Grand Canyon Monitoring and Research Center could weaken the very surveillance that tracks the river’s condition. Ecologist Larry Stevens of the Springs Stewardship Institute warned that a lapse of attention means degradation of the ecosystem.
For now, the chub’s comeback and its vulnerability share the same river. The rebound was real, and so is the threat that the cold-water protection that helped make it possible will not return.
Sources
1. Erik Stokstad, “Halted dam releases threaten Colorado River ecosystems,” Science (AAAS), August 11, 2026. DOI: 10.1126/science.z8qk1yy. https://www.science.org/content/article/halted-dam-releases-threaten-colorado-river-ecosystems
2. Associated Press via KJZZ, “Officials won’t release cool water from Colorado River reservoir to help humpback chub,” August 7, 2026. https://www.kjzz.org/politics/2026-08-07/officials-wont-release-cool-water-from-colorado-river-reservoir-to-help-humpback-chub
3. Arizona Republic, “Lake Mead hits new record low water level,” August 7, 2026. https://www.azcentral.com/story/news/local/arizona-environment/2026/08/07/lake-mead-new-record-low-water-level/91218668007/
4. USGS Southwest Biological Science Center, “Humpback chub rebound: USGS science uncovers,” July 24, 2026. https://www.usgs.gov/centers/southwest-biological-science-center/news/humpback-chub-rebound-usgs-science-uncovers
5. Bureau of Reclamation, LTEMP program page and Glen Canyon Dam operations. https://www.usbr.gov/uc/progact/amp/ltemp.html ; https://www.usbr.gov/uc/water/crsp/cs/gcd.html
6. Circle of Blue, “U.S. government orders emergency actions to protect Glen Canyon Dam,” April 2026. https://www.circleofblue.org/2026/supply/u-s-government-orders-emergency-actions-to-protect-glen-canyon-dam