DURHAM, North Carolina -- A Duke University-led study of changes in the floor of Lake Powell, the nation's second-largest reservoir, found that during a period of prolonged drought, massive transfers of sediment from the perimeter of the reservoir into its deepest sections occurred, increasing the lake's capacity to hold usable water by three percent.
A paper based on the research was published in the November issue of the peer-reviewed journal Geology.
"We found that sediment accumulation into parts of the lake during a six-year period of drought increased 10 to 100 times what the lake's long-term averages were," says Lincoln Pratson, associate professor of sedimentary geology at Duke's Nicholas School of the Environment and lead author on the study.
"The total transfer of sediment from the perimeters of the lake into its deep waters equated to approximately 22 years of average sediment supply," he says. "Intuitively, you'd think this redistribution would decrease the lake's capacity to hold usable water. But because much of the sediment was moved from the shallow edges of the reservoir – where usable water can be held – to the reservoir's deepest sections below its outtake channels, where the water is unusable, the inverse was true."
Pratson and a team of multi-institutional researchers conducted three bathymetric surveys between 1999 and 2005 to map the bottom of Lake Powell, a manmade reservoir in southern Utah. The surveying took place during a period of historic drought. The team wanted to learn what effects the drought and the subsequent draw-down of water levels in the lake – which had dropped roughly 45 meters – were having on sediment infilling, which can affect a reservoir's capacity for holding usable water and, over time, may reduce its usable lifespan.
They found that although the inflow of water to Lake Powell from the Colorado, San Juan and other upstream rivers was halved by the drought, it still caused massive erosion on the perimeters of the lake, where water-level fall had exposed poorly consolidated delta deposits.
The diminished river flows crossing these deltas became so choked with sediment that periodically they entered the lake essentially as submarine avalanches, carrying much of the sediment deep into the lake. Using acoustic imaging, Pratson and his team were able to track one underwater avalanche as it spilled down from a delta into the lake.
"As in natural basins, water and sediment supply to Lake Powell are coupled through the tie between regional climate and hydrology," Pratson says. "Actual observations of this coupling have been rare. Past studies have been largely based on geologic proxies of environmental change. In this instance, we actually were able to observe how a major climatic event has affected sediment infilling of a freshwater resource."
These observations "may also have relevance for the analogous study of the origin and timing of deep-sea oil and gas reservoirs along continental margins, which tend to form during climate-driven lowerings of sea level," he notes.
Pratson's co-authors on the study were John Hughes-Clark of the University of New Brunswick; Mark Anderson and Jesse Granet of the National Park Service; David Twichell of the U.S. Geological Survey; Ronald Ferrari of the Bureau of Reclamation; Charles Nittrouer and John Crockett of the University of Washington; and Thomas Gerber, a recent PhD graduate at the Nicholas School who is now at the ChevronTexaco Energy Technology Company.
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