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SEDIMENTATION IN PACIFIC NORTHWEST COASTAL STREAMS -- EVIDENCE FROM REGIONAL SURVEY OF BED SUBSTRATE SIZE AND STABILITY
Citation:
Kaufmann, P R., J. M. Faustini, AND D P. Larsen. SEDIMENTATION IN PACIFIC NORTHWEST COASTAL STREAMS -- EVIDENCE FROM REGIONAL SURVEY OF BED SUBSTRATE SIZE AND STABILITY. Presented at American Geophysical Union fall meeting, San Francisco, CA, December 8-12, 2003.
Description:
Excessive erosion, transport and deposition of sediment are major problems in streams and rivers throughout the United States. We examined evidence of anthropogenic sedimentation in Oregon and Washington coastal streams using relatively rapid measurements taken from surveys during summer low flows (probability sample, n= 104 reaches). We inferred bankfull channel dimensions from evidence at low flow and compared bed substrate geometric mean diameter (Dgm) with critical diameter (D*cbf) calculated from estimated bed shear stress at bankfull flow stage. We derived an index of Relative Bed Stability, RBS* = Dgm / D*cbf , modifying Dingman's (1984) formulation by adjusting bed shear stress for energy dissipated by large wood and channel irregularities. D*cbf averaged 56% of unadjusted values, and Dgm ranged from silt to boulder size. Log10[ RBS*] in streams relatively undisturbed by humans was generally between -0.7 and +0.5, showing approximate balance between observed and mobile substrates, but declined to low values (-1.5 to -3.0) in streams with streams with substantial riparian and basin disturbances. Streams draining soft sedimentary lithology showed greater apparent response to these disturbances than did those draining hard basalt lithology. The association of low RBS* with land disturbance, and the stronger absolute values of correlation of Dgm (negative) than D*cbf (zero or positive) with land disturbance in this region strongly suggest that activities including road building, land-clearing, logging and agriculture have augmented sediment supplies and increased fine sediments in stream beds.