Analysis of suspended-sediment concentrations and radioisotope levels in the Wild Rice River basin, northwestern Minnesota, 1973-98

Water-Resources Investigations Report 2001-4192

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Soil erosion and sediment loading to streams are concerns in the Red River of the North Basin (RRB) in Minnesota, North Dakota, South Dakota, and Manitoba, Canada. Soil erosion may reduce cropland fertility. Agricultural drainage ditches fill with eroded sediment over time, and require costly ditch maintenance. High suspended-sediment concentrations can also adversely affect aquatic ecosystems (Waters, 1995). Water utilities that use water from the Red River of the North (Red River) as a source of drinking water must spend more to treat water that has high sediment concentrations. Lake Winnipeg, in Manitoba, receives most of its tributary sediment loading from the Red River. Eutrophication in southern Lake Winnipeg due to sediment and nutrients (some portions of which are originally sediment-bound) is a concern. To better understand the dominant sources of sediment? channel processes or upland soil erosion?to streams in the RRB, the U.S. Geological Survey (USGS) in cooperation with the Legislative Commission on Minnesota Resources studied soils and suspended sediments of the tributary Wild Rice River Basin of northwestern Minnesota. The RRB is set in glacial-lake-bed, glaciofluvial, and morainal topography. Nearly all of the streams flow through glacial deposits or glacial lake-bed sedimentary deposits, and exhibit channel meanders, cut banks, and point bars, and often fairly turbid waters. Much of the RRB?particularly in the Red River Valley ?is cultivated cropland, and soil erosion from cropland also contributes to the sediment load in streams. It is widely accepted that sediment sources in streams in such settings are comprised of sediment that originates both from eroded soil and from erosion of stream-bank sediments (Colby, 1963). The relative amounts from these two sources in a given stream is seldom known. It is important for natural resource managers to gain a better understanding of sediment sources, so that management efforts can be targeted accordingly. The study objectives were to: (1) analyze suspended-sediment concentrations at stream sites within the Wild Rice River Basin for possible relations to streamflow, seasonality, and longterm trends, and calculate sediment loads; (2) determine the relative importance of soil erosion versus streambank erosion as potential sources of sediment to the Wild Rice River, based on activities of the fallout radioisotopes lead-210 (210Pb), cesium- 137 (137Cs), and, secondarily, beryllium-7 (7Be) in suspended sediments in transport and in potential sources; and (3) determine gross erosion rates from upland source areas using radioisotope analysis of soil cores from cultivated and undisturbed settings. This report describes results related to objectives (1) and (2). Objective (3), an analysis of soil-core isotopic inventories measured to determine historical upland erosion rates, is being addressed separately (C.J. McCullough,U.S. Geological Survey, written commun., 2001). This report summarizes suspended-sediment concentration data collected during 1973?98. Radioisotope samples were collected during 1998. Sampling for isotopic activities was targeted toward runoff events, with minimal sampling during base flow.

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Analysis of suspended-sediment concentrations and radioisotope levels in the Wild Rice River basin, northwestern Minnesota, 1973-98
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Water-Resources Investigations Report
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iv, 21 p. : ill., maps ; 28 cm.