Lake Powell, a key reservoir in the Colorado River Basin, has lost 4 percent of its storage capacity since 1986, according to a new report from the U.S. Geological Survey (USGS) and the Bureau of Reclamation.

The loss is largely due to a buildup of sediments that have been continuously transported by the Colorado and San Juan rivers into the reservoir, according to the report. Monday’s storage capacity update follows previous such evaluations from 1986 and 1963, representing a 6.79 percent loss since the earliest survey.

The Colorado River system — which includes both the nation’s largest reservoir, Lake Mead, and Lake Powell, the second largest — provides water to approximately 40 million people, irrigates 5.5 million acres of agriculture and has the capacity to produce more than 4,200 megawatts of electricity from hydropower.

“The Colorado River system faces multiple challenges, including the effects of a 22-year-long drought and the increased impacts of climate change,” Assistant Interior Secretary for Water and Science Tanya Trujillo said in a statement.

Lake Powell is located behind Glen Canyon Dam, extending just south of the Utah-Arizona border along the southern edge of Grand Staircase Escalante National Monument.

Its total storage capacity is now 25,160,000 acre-feet, according to surveys conducted by the USGS in 2017 and 2018.

That total represents a decrease of 1,833,000 acre-feet, or just under 7 percent, from 1963 capacity, and a decline of 1,049,000 acre-feet, or 4 percent, from 1986 capacity, the report found. The average annual loss from 1963 to 2018 amounts to about 33,270 acre-feet per year.

To conduct the surveys, USGS scientists used high-resolution multibeam bathymetry — a measure of underwater topography — and lidar — a remote sensing technology that uses laser pulses to measure ranges. They then combined the data to create a “topobathymetric digital elevation model,” which will now be used as management tool for the reservoir and the adjacent dam, according to the report.

While Lake Powell began steadily filling with water in 1963 and eventually became a popular destination for recreation, the reservoir has been facing serious challenges since the early 2000s, the authors warned in the report’s abstract.

“Severe drought and increases in water demand have resulted in a significant drop in reservoir elevation and stored water, prompting a heightened level of interest in the current state and future of Lake Powell,” they added.

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