You are here:
Using a relative bed stability index to define a reference condition for assessing anthropogenic sedimentation
Citation:
FAUSTINI, J., P. R. KAUFMANN, AND D. P. LARSEN. Using a relative bed stability index to define a reference condition for assessing anthropogenic sedimentation. Presented at American Geophysical Union Meeting, San Francisco, CA, December 15 - 19, 2008.
Impact/Purpose:
We developed 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 to assess anthropogenic sedimentation in streams.
Description:
We developed 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 to assess anthropogenic sedimentation in streams. LRBS is the log of the ratio of bed surface geometric mean particle diameter (Dgm) to the estimated 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 large wood. We hypothesized that human activities that augment sediment supply (particularly of fine sediments) without correspondingly increasing runoff or decreasing channel roughness should lead to reductions in LRBS as a result of textural fining of the streambed. Thus, LRBS values outside the range commonly observed in least-disturbed sites within a given region or class of streams could indicate potential human-caused sedimentation impacts. We tested the LRBS index using EMAP data from the Pacific Northwest Coast (PNW) and Mid-Atlantic regions of the United States. In both regions, LRBS was strongly inversely related to measures of anthropogenic disturbance intensity both at the watershed scale and in local riparian zone. In the PNW, 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. Correlations between Dgm and land disturbance were stronger than those observed between D*cbf and land disturbance in both regions, 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 some streams (e.g., volcanic drainages in the PNW) may have resulted in part from anthropogenic increases in bed shear stress.