Sediment in rivers comes from erosion of the landscape as well as the erosion and collapse of the banks themselves. Just how much each source contributes to a river – and how it affects the flow and path of that river – is the subject of research by Peter Whiting, professor of geological sciences at Case Western Reserve University.
Taking measure of certain radionuclides found in the soil, including beryllium and lead, at various points along a 423-km-long section of the Yellowstone River, Whiting has determined how much of the sediment in the Yellowstone came from runoff and how much came from the streambanks. For example, streambank erosion contributes approximately 50 percent of the sediment at measurement sites up-river, increasing to 89 percent at Billings, Mont. In river basins where significant portions of the surrounding landscape are used for agriculture or forestry, the percentage of sediment coming from streambank erosion drops below 50%.
Whiting will present his findings on October 6, at the 2008 Joint Meeting of the Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, and Gulf Coast Association of Geological Societies in Houston..
Radionuclides occur in soil both from natural processes and as fallout from nuclear testing. Beryllium and lead are found in greater concentrations at the surface of the soil. All the beryllium will be found in the top two centimeters of the surface soil but lead will be found to greater depth.
Beryllium and lead have markedly different half-lives. Lead has a 20-year half life, while that of Beryllium is only 53 days. Comparing the activities of both elements in the river's suspended sediment to the surrounding landscape and streambanks helps provide a detailed profile of where the sediment originates.
"We need to understand the sources of the sediment in our rivers if we want to address stewardship of our rivers," said Whiting.
For instance, fine sediment carried into rivers can cloud the water and can choke out freshwater bugs and fish that require cleaner water. Fine sediment deposited on the stream bottom can smother eggs laid by fish including salmon and walleye. To preserve these populations of fish, we often try to rehabilitate streams by reducing the amount of sediment supplied to the stream. But to try to reduce the supply, and one needs to understand whether it is activities eroding the landscape – urbanization, farming, or timbering – or it is the streambanks that are the primary cause of the problem.
"In using radionuclides as markers in our research, we are helping to develop new tools for the advancement of soil and river stewardship," said Whiting.
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