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Watching the river flow
California’s Death Valley, a sun-baked desert that averages just two inches of rain a year, may be the last place one would expect to find a scientist who studies rivers. But it’s this very scarcity of water that makes the locale fascinating to Noah Snyder, a Boston College assistant professor of geology and geophysics, who specializes in how rivers respond to changes in their surroundings—both to gradual shifts of climate and topography and to sudden disruptions such as dam construction.
The bedrock in Death Valley is relatively soft and covered with very little vegetation and soil. As a result, the impact of a steady rain on the landscape is dramatic. Sudden flows of water spread sand and gravel across normally bone-dry washes, and the bedrock “almost literally melts,” says Snyder. The geologist is engaged in an ongoing research project that focuses on the effects of the 1941 diversion of a desert wash known as Furnace Creek, on the eastern side of Death Valley National Park. Because of the area’s sensitivity to rain, he says, changes have occurred across six decades that would take thousands of years to unfold in a less extreme ecosystem. Findings have been published in the February 2008 issue of the journal Geology.
The National Park Service diverted Furnace Creek Wash after two floods in 1939 and 1941 heavily damaged the downstream village of Furnace Creek. Since then, the narrow canyon of Gower Gulch, which runs west from the wash to the floor of Death Valley, has born the brunt of the flash floods that flow down from the Funeral Mountains every couple of years. “You could view the dam-building as ‘climate change,'” says Snyder, “because the narrow Gower Gulch was accustomed to one ‘climate,’ with very little flow and very little water, and then, after the diversion, it suddenly experiences a much greater flow of water and sediment.”
Working with Lisa Kammer, MS’05, Snyder tracked the transformation in Gower Gulch that ensued after the dam’s construction by analyzing five sets of aerial photographs taken between 1948 and 1995 by the U.S. Geological Survey, as well as the results of a 2005 airborne laser mapping survey sponsored by the National Center for Airborne Laser Mapping (a National Science Foundation research center), which recorded elevations in the wash and gulch every square meter; in 2005 Snyder and Kammer took their own on-the-ground measurements of channel width every 25 meters, and assessed the size of sediment grains in the channel bed.
They studied the area in sections, stretching from Furnace Creek Wash and a series of waterfalls just below the dam, through the upper and lower gulch to the alluvial fan where the gulch empties into the bottom of Death Valley. What they found, says Snyder, was that flowing water sliced deeply into the rock of the upper, steeper section of the gulch, but it spread more sediment in the gulch’s lower, flatter sections, widening the channel in the lower gulch by 33 feet on average, or 66 percent. (The upper gulch experienced some widening, too, expanding by an average of 20 feet, or 50 percent.)
“It’s interesting, because I think a lot of people would have assumed that the whole thing would just be eroding like crazy,” says Snyder. These results indicate how important the preexisting valley slope and initial scatterings of sediment are when a river’s normal flow is disturbed. “As soon as you start getting deposition of sediment, you’re not going to erode the bed,” Snyder explains. Instead, “the water flow attack[s] the channel walls.”
According to Kammer, who is now an environmental consultant based in the Boston area, construction of the Furnace Creek dam gave the researchers a rare “real-world laboratory”—isolated from extraneous influences (development, agriculture) that affect rivers elsewhere, and with historical data available to serve as a control. Scientists can use the findings to test the physics-based computer models currently employed to forecast long-range evolution of rivers and topographical change. Snyder has used such models in his studies of New England rivers impacted by reforestation, stream restoration, fish reintroduction, and dam removal, research that last year earned him an Early Career Development grant from the National Science Foundation.
Chris Berdik is a writer in Boston.
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