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The already shrinking Lake Powell just got 4% smaller than previously believed.
In a study released Monday, the U.S. Geological Survey determined sediments washing into Utah’s largest reservoir reduce its capacity by 33,270 acre-feet each year on average as the sediment settles on the lakebed.
That means the 186-mile long Powell has lost 1.05 million acre-feet of capacity since 1986, when the last sedimentation survey was completed, and 1.83 million, or 6.8% of its capacity, since the lake first began filling behind Glen Canyon Dam in 1963, according to the 34-page study.
To put that number in perspective, the amount of sediment that has washed into Lake Powell is nearly twice the volume that developers are proposing to dredge from Utah Lake and to build 18,000 acres of islands.
“It is vitally important we have the best-available scientific information like this report to provide a clear understanding of water availability in Lake Powell as we plan for the future,” said Tanya Trujillo, Assistant Secretary of the Interior for water and science. “The Colorado River system faces multiple challenges, including the effects of a 22-year-long drought and the increased impacts of climate change.”
Powell can now hold up to 25.1 million acre-feet of water, about twice the annual flow of Colorado River and its upper tributaries. But after two decades of drought and diminishing snowpacks on the Colorado Plateau, the lake level is 175 feet below full and remains at risk of falling to a point where Glen Canyon Dam’s hydropower turbines can no longer generate electricity. The lake is less than a quarter full.
Lake Powell is the centerpiece of a vast water storage system overseen by the U.S. Bureau of Reclamation on the Colorado River, which provides water to 40 million people, irrigates 5.5 million acres and generates electricity.
During the most recent survey of Lake Powell, USGS scientists produced a topographic map of the lake bed using high-resolution multibeam bathymetry and lidar. The data were combined to create a continuous representation known as a topobathymetric digital elevation model, or TBDEM, indicating where sediments have accumulated.
Sedimentation is a problem vexing reservoirs all over southern Utah. Instead of getting flushed down streams and rivers, this fine sandy material is settling onto lakebeds, and will eventually render many reservoirs inoperable unless the sediments are dredged.
Wide Hollow near Escalante and Millsite near Ferron present two of the most extreme examples.
There is no bigger sediment trap in Utah than Glen Canyon Dam, which blocks sand from reaching Grand Canyon where it would naturally form beaches, sandbars and habitat for riparian vegetation and animals. Instead these particles are piling up where the Green, San Juan, Escalante, aptly named Dirty Devil and smaller Colorado tributaries hit Lake Powell’s slack water.
These deposits are now jokingly called the Dominy Formation in honor of Floyd Dominy, who ran the Bureau of Reclamation at the time of Glen Canyon Dam’s construction.
With Powell’s receding lake level exposing vast reaches of lake bed, these sediment beds make for an almost otherworldly landscape, punctuated by the barkless trunks of long-dead cottonwood trees.