Researchers with the science journal Nature modeled sediment buildup in more than 57,000 U.S. reservoirs. Collectively, they estimate the reservoirs have lost roughly 8% of their original storage capacity. But the losses are much greater in many individual reservoirs: about two out of five have lost more than one-quarter of their storage capacity.
The Colorado, Columbia and Rio Grande river basins are among the hardest hit.
Abigail Eckland, a researcher at Louisiana State University, New Orleans, who led the study, said those losses can be especially important in drier climates, like out West.
“In the West, we have more environments that are arid to semi-arid,” Eckland said. “We build reservoirs so that we can store water during wet years, and then use that water during dry years.”
Rivers naturally carry sediment downstream. But when water enters a reservoir, it slows down, allowing dirt, sand and other material to settle to the bottom. Over time, that buildup takes up space that otherwise could hold water.
That can become particularly important during prolonged droughts, Eckland said, when Western reservoirs may already be at lower levels and have fewer opportunities to refill. Water also tends to remain in Western reservoirs longer than in wetter parts of the country.
Wildfire could also add to the problem.
When fires burn vegetation and roots that help hold soil in place, hillsides can become more vulnerable to erosion. Some burned soils can also repel water, allowing rainfall to run off more quickly rather than soaking into the ground, which can lead to flash floods and mudslides.
That means storms over burned landscapes can wash more soil and debris into rivers — and eventually reservoirs.
“Increased wildfire over time can certainly increase sediment yields in a basin,” Eckland said, “and that would lead to higher reservoir sedimentation rates.”
The study itself did not directly model the effects of climate change. Its projections assume historical sediment rates continue into the future. Eckland said changes in wildfire and extreme precipitation could alter those rates.
Reservoir managers do have options to recover some lost storage, however. They can remove built-up sediment through dredging or flush it through dams and back into rivers. Other projects can divert sediment around reservoirs. Which approach works best, Eckland said, depends on the dam, river and potential effects downstream.
This story was produced by the Mountain West News Bureau, a collaboration between Boise State Public Radio, Wyoming Public Media, Nevada Public Radio, KUNR in Nevada, KUNC in Northern Colorado, KANW in New Mexico, Colorado Public Radio and KJZZ in Arizona as well as NPR, with support from affiliate newsrooms across the region. Funding for the Mountain West News Bureau is provided in part by the Corporation for Public Broadcasting and Eric and Wendy Schmidt.
