Around California, the country and the world, reservoirs are silently filling with sediment, and only a few people are thinking about it. Among them is Tim Randle, a civil engineer with the United States Bureau of Reclamation’s Sedimentation and River Hydraulics Group.
“We used to be gaining water storage capacity with dam building,” said Randle, who is based in Denver. “Now, around the world, the pace is slowing down as sediment builds up. This is true in the U.S., too.”
In the Paonia Reservoir, completed in 1962 in Gunnison County, Colorado, for example, the dam’s outlet was built 60ft off the lake’s bottom. Now, Randle says, the bottom of the lake is above the outlet. The outlets in many other small reservoirs have become clogged with sediment, requiring expensive dredging or even the removal of the dams. Other times, boat ramps and marinas get buried and filled in. According to Randle, about 35 percent of the reservoirs managed by his agency have been surveyed for sediment fill. With most reservoirs, however, how much capacity has been lost is a matter of educated guesswork.
“Lake Powell probably has 1 million acre-feet less water than what we think,” Randle says.
And now the problem is predicted to get much worse.
According to new research from the U.S. Geological Survey, in many regions erosion rates are now accelerating thanks to wildfires and climate change. The western U.S., which relies on reservoirs for vital water storage and flood control, will be particularly impacted.
The problems of reservoir sedimentation have been at least somewhat understood for centuries, and most dams in the United States have been built with average rates of upslope erosion factored into the placement of the outflow pipes. In more erosive watersheds, Randle says, outlets are built higher off the soon-to-be reservoir’s bottom. In watersheds where soil naturally moves downslope at a slower pace, dam intakes can be built at a lower elevation.
Authors of the USGS study, which was published September 7 in the journal Geophysical Research Letters, wrote that projected increases in the frequency and magnitude of wildfires will double the rates of sedimentation in one-third of 471 large watersheds in the western U.S. within the next 33 years. In almost nine out of 10 of the watersheds assessed, sedimentation could increase by at least 10 percent, the researchers warned. In some watersheds, the researchers predict, erosion and sedimentation could increase by 1,000 percent. Climate change, they concluded, is the underlying culprit.
Randle says climate change – which may already be increasing the intensity of droughts and, in turn, wildfires – is driving a vicious cycle whereby sedimentation rates increase. This reduces reservoir storage space, “which leaves you less prepared for the next drought.”
Joel Sankey, a coauthor of the study and a USGS research geologist, says portions of the Cascade Range in Washington, Oregon and California, as well as parts of the Sierra Nevada, are likely to experience particularly dramatic increases in fire damage and subsequent erosion. Since the root systems of trees and shrubs literally hold mountainsides together, when such vegetation dies en masse – as it can in destructive wildfires – vast quantities of soil may subsequently erode into streams and rivers. Sankey says the first year after a fire presents the greatest erosion hazard.
But impacts may last for years. In California’s Sierra Nevada foothills, much of the acreage incinerated by the 2014 King fire remains mostly bare.
“We’re just beginning to see the small plants return, and this is year three after the fire,” said Andy Fecko, the Placer County Water Agency’s director of resource development. “We expect it will be five to 10 years before the watershed recovers.”
The King fire burned 98,000 acres, about 40,000 of which were within the Placer County Water Agency’s drainage area. This has caused an “exponential” increase in local erosion, Fecko said.
Elsewhere, water and forest managers well know the correlation between fires and post-burn erosion. For example, more than 1 million cubic yards of sediment entered Strontia Springs Reservoir, a major supply lake for Denver Water, following the Hayman fire, which burned 138,000 acres in Colorado in 2002.
Randle says the preferred course of action in mitigating sedimentation is preventative – especially protecting upslope areas against erosion, which results mostly from fires, logging, farming and development. Once sediment enters reservoirs, it is more challenging to deal with. He says dam operators may open the release gates to allow muddy water that has gushed into a reservoir after a storm to exit the system before the sediment can sink to the bottom.
Once sediment has settled to the bottom, the most effective means of removal is dredging, which can be costly.
“It’s much more expensive to dredge out your reservoirs after a fire than it is to take preventive action, like reducing fuel loads and restoring forests,” said Jason Kreitler, a research geographer with the USGS and a coauthor of the study.
Dredging a reservoir can cost as much as $60 per cubic yard of material, according to Randle. Some operations remove hundreds of thousands, and even millions, of cubic yards of material. Finer-grained silt and clay is cheaper to deal with.
“Sand and gravel is coarser and tends to do more damage to the dredging equipment,” he noted.
Denver Water has spent $27 million removing debris and sediment from Strontia Springs Reservoir, according to a June report. The Los Angeles County Public Works plans to spend $190 million dredging four reservoirs impacted by sediment from the 2009 Station fire, according to a 2013 report from the U.S. Department of the Interior.
Fecko says his agency has removed 44,000 cubic yards of material from Ralston Afterbay, a small hydroelectric reservoir on the Middle Fork of the American River, at a cost of $2.2 million. Still, the reservoir has lost about 50 percent of its storage capacity thanks to long-term sedimentation, he says.
Besides lost reservoir storage, there are other impacts from sedimentation. Downstream from dams, rivers become depleted of gravel – essential for spawning salmon. In fact, gravel must be poured regularly into many spawning areas to mimic gravel deposition cycles lost with dam construction. Beach erosion, too, has become a chronic problem in many coastal regions where dammed rivers no longer carry inland sands to the ocean. Japan and Taiwan, Randle says, are both experiencing rapid beach loss, as is California.
Randle, at the Bureau of Reclamation, thinks all water agencies and local governments would be wise to take a proactive stance against sediment entering reservoirs.
He likens sedimentation of reservoirs to clogged arteries.
“You might not have symptoms until you have a heart attack,” he noted. With sedimentation of reservoirs, he explains, the impacts do not appear until the sediment clogs a dam’s outflow or until the problem literally surfaces.
“And for every reservoir in the world, it’s not a matter of if this will happen, but when,” Randle said.