From Elephant Butte To Oregon, Reservoirs Filling With Silt - Rates Vary, But Overall Plans Lacking

[hatrack]

Gary Esslinger, manager of Elephant Butte Irrigation District in southern New Mexico, spends as much time moving silt as he does water. Elephant Butte Reservoir, built in 1915, is fed by the naturally muddy Rio Grande, which drains 28,000 square miles of easily eroded desert in two states. Sediment has claimed 600,000 acre-feet of its 2.6 million acre-foot capacity. "If I could create a bumper sticker," Esslinger says wryly, "it would say, 'Silt Happens.'"

The sediment clogs canals, pipelines and farm fields. It has filled 33 small flood control dams below Elephant Butte, effectively rendering them useless. The district -- which supplies some 7,900 farmers -- maintains a fleet of excavators, dozers and dumptrucks, but Esslinger is running out of places to move dirt. He encourages developers to haul it away for fill, but demand remains low. "It's just going to become a bigger and bigger problem as these (dams) get older," he says.

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Utah is one of the few Western states that have even attempted to assess sedimentation. In a March 2010 report, the state's Department of Water Resources estimated that in 40 years, Utah's total storage capacity will have declined 25 percent. Its reservoirs lose about 12,340 acre-feet a year to sedimentation, yet the state needs about double that amount annually in additional supply to keep up with population growth. The reservoirs "cannot be considered renewable resources unless sedimentation is adequately addressed," the report states. But Utah was able to find data for only 18 of its 133 reservoirs larger than 1,000 acre-feet. Nationally, the state of knowledge is equally poor. John Gray, a hydrologist at the U.S. Geological Survey in Reston, Va., manages the nation's only large database on reservoir sedimentation, which includes surveys of 1,824 large and small reservoirs across the country compiled by the Soil Conservation Service, now the Natural Resources Conservation Service.

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Most large reservoirs have centuries to go, but small ones may have only decades, and many have already filled completely. On the Stanford University campus in California, sediment has reached the brim of Searsville Dam. Meanwhile, the Matilija Dam on California's Ventura River, which was built in 1948 for water supply and flood control, now provides neither. A single storm in 1969 filled 27 percent of the reservoir with sand and mud; it's currently 90 percent filled with sediment. There is agreement to remove the dam, but finding a place to move the accumulated muck remains an obstacle. "Every reservoir is headed towards that condition, just at different rates," says Baskin. A variety of solutions are possible; none are cheap. Sedimentation can be reduced by improved land management upstream to minimize erosion. Mechanical fixes are also available, from dredging to adding diversion structures upstream to separate and transport sediment elsewhere. Dams can also be retrofitted with new gates so sediment can be flushed out. At small reservoirs, dams are often raised, recovering lost capacity, but only for a short time.

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https://www.hcn.org/issues/43.6/muddy-waters-silt-and-the-slow-demise-of-glen-canyon-dam/sedimentation-a-building-problem-in-the-wests-reservoirs

[thereismore]

1. That's for messing with nature. nt