A new report released Tuesday by the U.S. Geological Survey confirms Lake Powell has lost 4% of its potential storage capacity since 1986, when the last survey was completed.
The report also shows a nearly 7% loss since 1963, when the diversion tunnels of Glen Canyon Dam were closed and the reservoir began to fill up.
Officials with USGS say the loss is largely due to sediments continuously transported by the Colorado and San Juan rivers settling to the bottom of Lake Powell.
The storage capacity has been calculated only twice before the current study.
During the most recent survey, USGS scientists used high-resolution multibeam and lidar technology to create the equivalent of an underwater topographic map.
Lake Powell is a key water storage unit on the Colorado River, providing water to approximately 40 million people in western states.