HR: 1340h
AN: H53B-1245    [Abstracts]
TI: Geomorphic Effects of Boulder Placement on Gravel Capture and Retention in a Regulated Reach of the North Umpqua River, OR.
AU: * Stallman, J
EM: jay@stillwatersci.com
AF: Stillwater Sciences, 850 G Street,, Suite K, Arcata, CA 95521 United States
AU: Braudrick, C
EM: xian@berkeley.edu
AF: University of California Berkeley, Department of Earth and Planetary Science, McCone Hall, Berkeley, CA 94720-4767 United States
AU: Pedersen, D
EM: dirk@stillwatersci.com
AF: Stillwater Sciences, 850 G Street,, Suite K, Arcata, CA 95521 United States
AU: Cui, Y
EM: yantao@stillwatersci.com
AF: Stillwater Sciences, 850 G Street,, Suite K, Arcata, CA 95521 United States
AU: Sklar, L
EM: leonard@sfsu.edu
AF: San Francisco State University, Department of Geosciences, 509 Thornton Hall 1600 Holloway Ave., San Francisco, CA 94132 United States
AU: Dietrich, B
EM: bill@geomorph.berkeley.edu
AF: University of California Berkeley, Department of Earth and Planetary Science, McCone Hall, Berkeley, CA 94720-4767 United States
AU: Real de Asua, R
EM: raf@stillwatersci.com
AF: Stillwater Sciences, 850 G Street,, Suite K, Arcata, CA 95521 United States
AB: Hydroelectric projects in the mountainous western Cascades often occur in steep, confined channels where salmonid spawning habitat is limited to gravel deposits forced by planform curvature, channel width changes, and flow separation associated with large bedrock and boulder obstructions. The paucity of gravel deposition in steepland channels may be exacerbated in regulated rivers where sediment trapping by impoundments reduces coarse sediment supply to downstream reaches. Placing boulders to capture and retain gravel may be an effective approach to enhancing spawning habitat in these settings. To better understand the potential use of boulders as a tool for enhancing spawning habitat, three experimental designs were tested in a 0.6-mile bypass reach of the North Umpqua River, OR. The bedrock-confined study reach has an average slope of 0.013 and plane-bed morphology with coarse cobble substrate, abundant marginal boulders, and small associated patches of sand and gravel. Experiments involved (1) placement of boulder clusters, (2) gravel augmentation and placement of boulder clusters, and (3) gravel augmentation alone. Boulder clusters were designed to promote scour and deposition during floods with a 5-10 year recurrence interval. Boulders were typically placed obliquely upstream at locations where existing hydraulics favored gravel deposition. Monitoring from 2002 to 2004 occurred prior to implementation, immediately following implementation, and following winter high flows. Sites were monitored using high-density topographic surveys, low-altitude aerial photography, facies mapping, pebble counts, scour cores and chains, and marked rocks. Stage heights were monitored using pressure transducers at the upstream and downstream ends of the study reach, and flood recurrence interval was assessed using a nearby USGS gauge. The arrangement of boulder clusters was modified after the first year of monitoring to improve gravel capture and retention. Peak flow during the two-year monitoring period had a recurrence interval of less than 1.5 years. Flows were insufficient to mobilize the bed as a whole, but did adjust bed surface texture and topography adjacent to boulder accumulations. Select sites captured and retained modest amounts of gravel even at the relatively low peaks experienced during 2003 and 2004. The effects of increasing coarse sediment supply will be tested in 2005 through the introduction of a large gravel pulse at the upstream end of the study reach.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1827 Glaciology (1863)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting