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Quantifying Sediment Loading From Stream Bank Erosion in Pastures

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org

Citation:  Watershed Management to Meet Water Quality Standards and Emerging TMDL (Total Maximum Daily Load) Proceedings of the Third Conference 5-9 March 2005 (Atlanta, Georgia USA) Publication Date 5 March 2005  701P0105.(doi:10.13031/2013.18086)
Authors:   Mark S Riedel, Kenneth N Brooks and Elon S Verry

Streams in the Nemadji River Watershed of east-central Minnesota are deeply incised in lacustrine clay and glacial till. Due to "recent" glacial activity (~10,000 years before present) and resultant geologic uplift, this region is quite erosive. Indeed, the Nemadji River is the largest source of fluvial sediments to Lake Superior (both on mass and per unit area basis). While natural land cover was dominated by coniferous forests, riparian areas were commonly used for grazing by the late 1800's. To determine the impacts of grazing on sediment yield, we compared morphology and sediment budgets of streams having forested and grazed riparian areas. We surveyed numerous cross-sections on representative study streams over a 3-year period. Grazed streams were generally larger, shallower (increased width depth ratio) and straighter (decreased sinuosity) than forested streams. The highly variable riffle-pool structure of forested riparian areas was lost to sedimentation and replaced by relatively uniform profiles in the grazed streams. Using watershed scale sediment budgets and channel morphology, we computed sediment budgets for each study reach. We then computed the minimum amount of erosion necessary to change the morphology of a "typical" forested stream to that of the grazed streams. We estimate that grazing only 0.01% to 0.1% of the riparian areas increased annual sediment yields by 2% to 9%, over a relatively short period (the grazed streams appear to have stabilized) following the introduction of cattle. While significant locally, these values are orders of magnitude lower than sediment yields from prevalent mass wasting.

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