Rolling Down the River

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Science  21 Oct 2011:
Vol. 334, Issue 6054, pp. 290-291
DOI: 10.1126/science.334.6054.290-c

All rivers naturally move loads of sediments, from coarse sand grains rolling along riverbeds to tiny clay and silt particles carried in suspension. When sediment load gets too high though, either naturally or from human activities, biodiversity suffers and water quality deteriorates. Identifying the sources of increased sediment loads, which can vary with such regional factors as land use and precipitation, is critical for implementing remediation strategies. As a case study for determining the mechanism of sediment transport on the scale of several watersheds, Belmont et al. integrated a number of data sources—including geochemical tracers, hydrologic field measurements, and remote sensing—from tributaries or lakes in Minnesota along North America's largest river, the Mississippi. Over the past 150 years, not only has the amount of fine-grained sediment increased by a factor of 10, but the sources have also changed. Historically, upland erosion of soil contributed most of the sediment load; however, the new data suggest that up to 70% of the sediment comes from the erosion of riverbanks and ravines themselves. The shift is probably a function of both natural and anthropogenic activity, including increased precipitation and extensive modification of drainage networks for agricultural purposes.

Environ. Sci. Technol. 45, 10.1021/es2019109 (2011).

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