Los Angeles: Lake Mead and Lake Powell, the two largest reservoirs of the United States on the Colorado River, have both fallen to record-low levels since they were filled, according to federal data.

US water officials and experts said the declines are increasing risks to water supplies, agriculture and hydropower across the Southwest.

Lake Powell fell to 3,519.91 feet (about 1,072.869 meters) above sea level on Saturday, according to provisional data published Sunday by the US Bureau of Reclamation. The reading was marginally below the previous record of 3,519.92 feet set in April 2023, and the bureau said provisional data are subject to revision.

Downstream, Lake Mead fell to as low as 1,039.85 feet Sunday afternoon, according to the bureau's report. Bureau records show that the previous record low was 1,040.58 feet, recorded in July 2022.

Jack Schmidt, director of the Center for Colorado River Studies at Utah State University, was quoted by CNN on Sunday as saying the basin was "in uncharted territory."

Lake Mead and Lake Powell were formed by the Hoover Dam and Glen Canyon Dam, respectively, and together they serve as the primary water storage and supply system for the Colorado River's 40 million downstream users across seven US states and Mexico.

The US Interior Department told a Senate hearing in June that long-term drought had reduced Colorado River system storage to about 34 per cent of capacity, reports Xinhua news agency.

Lake Powell, located in southern Utah and northern Arizona, is now about 9 metres above the 1,063.75-meter minimum power pool identified by the Bureau of Reclamation, the lowest elevation at which Glen Canyon Dam can currently generate hydropower.

According to data released by Grand Canyon Trust, the Glen Canyon Powerplant churns out about 5 billion kilowatt-hours of electricity every year, enough to power nearly half a million US households.

The Hoover Powerplant has a long-term average annual output of about 4 billion kilowatt-hours, said the Bureau of Reclamation, which warned in April that falling Lake Mead levels could further reduce generating capacity.

The bureau also announced then plans to move up to about 1.23 billion cubic meters of additional water from Flaming Gorge Reservoir, a large reservoir on a major tributary of the Colorado River, toward Lake Powell, and reduce annual releases from Lake Powell to Lake Mead by about 1.83 billion cubic meters as drought conditions worsened.

The federal government is also preparing new rules for operating the river system after current guidelines expire at the end of 2026.

On July 31, the Bureau of Reclamation released a Final Environmental Impact Statement outlining a preferred framework for the future management of Lake Powell and Lake Mead, with guidelines designed to remain in place through 2036.

Sharon Megdal, director of the Water Resources Research Center at the University of Arizona, told ABC News on Sunday that "it's kind of a critical situation on the river" because the two reservoirs had not faced such conditions since they were filled.