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ASSESSING STREAM BED STABILITY AND EXCESS SEDIMENTATION IN MOUNTAIN STREAMS
Citation:
Faustini, J. M. AND P R. Kaufmann. ASSESSING STREAM BED STABILITY AND EXCESS SEDIMENTATION IN MOUNTAIN STREAMS. Presented at American Geophysical Union fall meeting, San Francisco, CA, December 6-10, 2002.
Description:
Land use and resource exploitation in headwaters catchments?such as logging, mining, and road building?often increase sediment supply to streams, potentially causing excess sedimentation. Decreases in mean substrate size and increases in fine stream bed sediments can lead to increased frequency of bed scour, while accumulation of fine sediments in the interstices of coarse bed particles can adversely impact salmon spawning habitat and reduce habitat availability for benthic organisms. We are testing an index of relative bed stability (RBS), based on reach-scale synoptic stream surveys, that is calculated as the ratio of the observed geometric mean particle diameter to the estimated critical diameter at bankfull flow after adjusting for shear stress losses due to channel morphology and large woody debris (LWD). We hypothesize that in watersheds not altered by human disturbances, transport capacity should be in rough equilibrium with sediment supply and RBS should be close to unity. In streams where human activity has substantially augmented sediment supply, we expect that textural fining may occur, leading to lower RBS values. However, downstream trends of decreasing slope and particle size and increasing sediment supply might lead to systematic downstream trends in RBS, and variations in local channel characteristics could cause variability in calculated RBS values. To test whether RBS is useful as an indicator of textural fining in response to anthropogenic disturbance, we sampled streams in watersheds spanning a wide range of disturbance intensity (high, medium and low) in two sub-regions of contrasting lithology in the northern Coast Range of Oregon and in the mid-Atlantic U.S. In each watershed we sampled 3 closely-spaced main stem reaches (30-50 km2 drainage area) and 3 reaches in one or more smaller tributaries (5-10 km2) to assess local variability and within-basin longitudinal trends in RBS relative to variation between watersheds with different land use intensity. Preliminary results show the predicted association between land use and RBS in watersheds underlain by erodible rocks, but not in those underlain by resistant rocks. RBS was not a function of stream size in our sample.