HR: 0800h
AN: H51E-0411    [Abstracts]
TI: Sediment Dynamics in the Upper McKenzie River Basin, Central Oregon Cascade Range
AU: * Stallman, J D
EM: jay@stillwatersci.com
AF: Stillwater Sciences, 850 G Street, Suite K, Arcata, CA 95521 United States
AU: Bowers, R J
EM: ronna@stillwatersci.com
AF: Stillwater Sciences, 850 G Street, Suite K, Arcata, CA 95521 United States
AU: Cabrera, N C
EM: natalie@stillwatersci.com
AF: Stillwater Sciences, 850 G Street, Suite K, Arcata, CA 95521 United States
AU: Real de Asua, R
EM: raf@stillwatersci.com
AF: Stillwater Sciences, 2855 Telegraph Ave, 400, Berkeley, CA 94705 United States
AU: Wooster, J K
EM: wooster@stillwatersci.com
AF: Stillwater Sciences, 2855 Telegraph Ave, 400, Berkeley, CA 94705 United States
AB: Reference and current sediment budgets were developed to evaluate the extent to which hydroelectric dams alter sediment dynamics in the upper McKenzie River basin of central Oregon. The 647 km² study area straddles the western boundary of the High Cascades graben separating the High Cascades and Western Cascades geologic terrains. Permeable Quaternary volcanics forming the low-gradient High Cascades plateau promote surface hydrologic disconnection, nearly constant discharge controlled by groundwater emergence, and low sediment yield. In contrast, deeply weathered Tertiary volcanics, rugged topography, and a dense network of steep channels in the Western Cascades terrain promote peaked storm responses and high sediment yield by deep-seated mass movement, debris slides, and debris flows. Three independent estimates of sediment yield (application of published surface process rates, extrapolation of regional suspended load and bedload flux rates, and extrapolation of reservoir sedimentation rates) illustrate the dominant role of geologic terrains in determining the longitudinal pattern of sediment supply to the McKenzie River. Average reference yields from High Cascades and Western Cascades sources were 9 t km^-2y^-1 and 200 t km^-2y^-1, respectively. Downstream of Trail Bridge Dam, High Cascades sources (241 km²) account for 12% of the total reference yield, while Western Cascades sources (67 km²) account for 62%. Estimates of current sediment yield illustrate the offsetting effects of reservoir sediment trapping and accelerated yield related to forest management. Average current yields from High Cascades and Western Cascades sources were 17 t km^-2y^-1 and 300 t km^-2y^-1, respectively. Current yield to the McKenzie River arm of Trail Bridge Reservoir (42 km² sourced in High Cascades terrain) was 17 t km^-2y^-1, while current yield to Smith Reservoir (48 km² sourced in Western Cascades terrain) was 251 t km^-2y^-1. The relation between hydroelectric project effects and forest management are most apparent in the McKenzie River at Deer Creek, where current yield from Deer Creek (29% higher than reference yield) may largely compensate for sediment trapping in Smith and Trail Bridge reservoirs. The extent to which accelerated yield from Deer Creek moderates the geomorphic effects of trapping in upstream reservoirs is, in part, a function of storage changes in the alluvial reach of lower Deer Creek. Mass balance between average annual bedload transport capacity (22,000 t y^-1 and 13,000 t y^-1 at the upper and lower ends of the reach, respectively) and estimated coarse sediment supply (4,500 t y^-1) suggests that sediment input from Deer Creek to the McKenzie River is relatively insensitive to storage changes. Since hydroelectric dams and forest management likely decrease the coarse:total sediment ratio, the geomorphic effects of reservoir sediment trapping and accelerated tributary sediment yields depend on the current grain size distribution of the sediment supply relative to that stored in upstream reservoirs.
DE: 1815 Erosion
DE: 1825 Geomorphology: fluvial (1625)
DE: 1826 Geomorphology: hillslope (1625)
DE: 1861 Sedimentation (4863)
DE: 1862 Sediment transport (4558)
SC: Hydrology [H]
MN: Fall Meeting 2005