HR: 14:40h
AN: H32F-04    [PDF]
TI: Sedimentation in Pacific Northwest Coastal Streams -- Evidence from Regional Surveys of Bed Substrate Size and Stability
AU: * Kaufmann, P R
EM: kaufmann.phil@epa.gov
AF: U.S. Environmental Protection Agency, 200 SW 35th Street, Corvallis, OR 97333 United States
AU: Faustini, J M
AF: Dept. of Fisheries and Wildlige, Oregon State University, c/o USEPA, 200 SW 35th Street, Corvallis, OR 97333 United States
AU: Larsen, D P
AF: U.S. Environmental Protection Agency, 200 SW 35th Street, Corvallis, OR 97333 United States
AB: 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, based on a modified Shield's criterion for incipient motion that includes adjustments to bed shear stress for energy dissipated by large wood and channel irregularities. D*cbf averaged 56 percent 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 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.
DE: 1803 Anthropogenic effects
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1860 Runoff and streamflow
SC: Hydrology [H]
MN: 2003 Fall Meeting