ARIZONA — Officials closed San Carlos Reservoir indefinitely on June 5, 2026, after declining water levels dropped its capacity to less than 1 percent full and caused hypoxia that killed nearly all fish in the reservoir.

On May 22, 2026, the reservoir contained 389 acre-feet of water, which represented less than 1 percent of its full capacity. Required water releases for downstream agriculture reduced the water volume to less than 400 acre-feet by June 2026. In June 2023, the reservoir was approximately 60 percent full. The Coolidge Dam created the reservoir, which is one of Arizona's largest bodies of water when at full capacity.

The San Carlos Recreation and Wildlife Department warned, "Decomposing fish may pose health risks to people attempting to boat or fish." Fish species previously found in the reservoir included largemouth bass, black crappie, bluegill, channel catfish, flathead catfish, brown trout, and rainbow trout.

Factors contributing to the low water levels included a diminished snowpack in the Gila River watershed. Mountain snowpack in the region reached 2 percent of the 1991-2020 March median in 2026. This reduced snowpack decreased April 2026 streamflow in the Gila River watershed to 39 percent of normal levels.

San Carlos Reservoir has run dry at least 20 times since its filling in 1930. Major fish kills also occurred in 1976 and 2018. The 1976 fish kill resulted in the death of over 5 million fish, and the reservoir's ecosystem required five years to recover. Vegetation along the exposed shoreline and river channel at the reservoir includes tamarisk, willow, cottonwood, sedges, and grasses.

Severe drought conditions affect much of the Gila River headwaters in New Mexico. Despite the drought, a seasonal monsoon outlook issued in May 2026 projected a 33 to 50 percent chance of above-average rainfall in the region during the summer. El Niño conditions also strengthened in the central and eastern equatorial Pacific during late spring 2026. El Niño conditions can increase the likelihood of heavy rainfall in the U.S. Southwest.