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Assessing Relative Bed Stability and Excess Fine Sediments in Streams

Philip R. Kaufmann 1, David P. Larsen 1, and John M. Faustini 2

1 U.S. Environmental Protection Agency, ORD, Corvallis, OR 97333
2 Oregon State University, Corvallis OR, 97333

Excess fine sedimentation is recognized as a leading cause of water quality impairment in surface waters in the United States. We developed an index of Relative Bed Stability (RBS) that factors out natural controls on streambed particle size to allow evaluation of the role of human activities in stream bed sedimentation. This index is designed for rapid synoptic regional surveys such as the EPA's EMAP. RBS is calculated as the ratio of the geometric mean diameter of particles on the stream bed (Dgm) to the bankfull critical diameter (Dcbf), which is the largest particle size mobilized by bankfull flows that occur every 1 to 3 years. We adjust Dcbf for large wood and channel roughness that diminish stream erosive power. We examined evidence of anthropogenic sedimentation in a probability sample of 104 Pacific Northwest streams using summer low flow measurements. Log10[RBS] values between -0.7 and +0.6 in streams with low human disturbance suggested approximate balance between sediment supply and transport; low values (-1.5 to -3.0) indicated excess fine sediment in streams with substantial riparian and basin disturbances. Streams draining soft sedimentary lithology showed greater response to these disturbances than did those in hard basalt. Stronger correlations between land disturbance and Dgm (negative) than with Dcbf (zero or positive) suggest that land use activities have augmented sediment supplies and increased streambed fine sediments. RBS has strong potential for use in regional monitoring and assessment, as well as for 303(d) listing of streams with excess fine sediments.

Keywords: sediment, physical habitat, streams, human disturbances, land use, riparian condition.

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