After an exceptionally poor snow season, one of Arizona’s largest reservoirs shrank to less than 1% full, wiping out nearly all its fish and leaving its recovery dependent on future rains.
The Gila River is a vital waterway in the American Southwest, supplying water for communities, agriculture, and wildlife as it flows from the snow-covered mountains of southwestern New Mexico into the desert landscapes of southwestern Arizona.
During years with healthy winter snowfall, the Mogollon Mountains and Black Range provide much of the river’s spring runoff. That snowmelt replenishes San Carlos Reservoir, which was created by the Coolidge Dam and, when full, ranks among Arizona’s largest reservoirs.
Historic Snowpack Shortfall Leaves Reservoir Nearly Empty
Conditions were dramatically different in 2026. A very dry winter left the snowpack across the Gila River watershed at just 2 percent of the 1991-2020 March median. With so little snow available to melt, April streamflow dropped to only 39 percent of normal.
After required water releases for downstream farms, the situation became even more severe. By June, San Carlos Reservoir contained fewer than 400 acre-feet of water.
Satellite images captured the dramatic transformation. A Landsat image taken on May 22, 2026 (above), shows the reservoir holding just 389 acre-feet of water, leaving it less than 1 percent full. By comparison, imagery from June 2023 (below) shows the same reservoir at roughly 60 percent capacity. Along the remaining water, vegetation including tamarisk, willow, cottonwood, sedges, and grasses lines the river channel and exposed shoreline.
Low Water Triggers Massive Fish Die-Off
As water levels continued to fall, officials indefinitely closed San Carlos Reservoir on June 5, 2026. The shrinking reservoir caused oxygen levels to fall to dangerously low levels, a condition known as hypoxia, killing virtually all of the fish.
The reservoir supported species such as largemouth bass, black crappie, bluegill, channel catfish, flathead catfish, and stocked fish including brown trout and rainbow trout.
The San Carlos Recreation and Wildlife Department also warned that decomposing fish could create health hazards for anyone attempting to boat or fish in the reservoir.
A History of Dry Reservoir Conditions
Although the current situation is severe, it is not without precedent. News reports indicate that San Carlos Reservoir has completely run dry at least 20 times since it first filled in 1930.
Even during the reservoir’s dedication, conditions were so dry that grass covered parts of the exposed lakebed. Humorist Will Rogers famously joked to President Calvin Coolidge, “If that was my lake, I’d mow it.”
Major fish kills have also occurred before, including in 1976 and 2018. Following the 1976 event, when more than 5 million fish died, the Gila Herald reported that the reservoir’s ecosystem needed five years to recover.
Monsoon Rains Could Bring Relief
Much of the Gila River’s headwaters in New Mexico remains in severe drought as part of an ongoing multi-year dry period, according to the U.S. Drought Monitor.
Still, the river’s flow naturally varies from year to year, leaving open the possibility of a recovery. Heavy rainfall during the summer wet season could quickly improve water levels.
A NOAA seasonal outlook issued in May 2026 projected a 33 to 50 percent chance of above-average rainfall across the region during the summer monsoon. At the same time, El Niño conditions strengthening across the central and eastern equatorial Pacific could further increase the likelihood of heavy rain in the U.S. Southwest.
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