Anthropogenic Sedimentation in Pacific Northwest Streams Inferred from Aquatic Habitat Survey Data

We evaluated anthropogenic sedimentation in U.S. Pacific Northwest coastal streams using an index of relative bed stability (LRBS^*) based on low flow survey data collected using the U.S. Environmental Protection Agency's Environmental Monitoring and Assessment Program (EMAP) field methods in a probability sample of 101 wadeable stream reaches. LRBS^* is the log of the ratio of bed surface geometric mean particle diameter (D_gm) to critical diameter (D^*_cbf) at bankfull flow, based on a modified Shield's criterion for incipient motion that explicitly accounts for reductions in bed shear stress resulting from channel form roughness due to pools and wood. LRBS^* ranged from –1.9 to +0.54 in streams within the lower quartile of human riparian and basin disturbance, and was substantially lower (–4.2 to –1.1) in streams within the upper quartile of human disturbance. Streams draining relatively erodible sedimentary lithology (sandstone, siltstone) showed greater reductions in LRBS^* associated with disturbance than did those having more resistant volcanic lithology (basalt) with similar levels of basin and riparian disturbance. Contrary to our expectations, smaller streams had lower LRBS^* than those with larger drainages at any given level of disturbance. In sedimentary lithology, high-gradient streams had higher LRBS^* than did low gradient streams of the same size and level of human disturbance, but the opposite was true in volcanic lithology. Correlations between D_gm and land disturbance were stronger than those observed between D^*_cbf and land disturbance, suggesting that land use has augmented sediment supplies and increased streambed fine sediments in the most disturbed streams. However, we also show evidence that some of the apparent reductions in LRBS^* in volcanic drainages may have resulted in part from anthropogenic increases in bed shear stress. The EMAP habitat survey data appear adequate to evaluate regional patterns in bed stability and sedimentation and their general relationship to human disturbances.