HR: 13:45h
AN: H32F-01 INVITED     [PDF]
TI: Hydraulic Control on the Spatial Distribution of Chinook Salmon Spawning Gravels in Central Idaho: Integrating Reach-Scale Predictions via Digital Elevation Models
AU: * Buffington, J M
EM: jbuff@uidaho.edu
AF: University of Idaho, Ecohydraulics Research Group, Department of Civil Engineering, 800 Park Blvd., Ste. 200, Boise, ID 83712 United States
AU: Isaak, D J
EM: disaak@fs.fed.us
AF: University of Idaho, Ecohydraulics Research Group, Department of Civil Engineering, 800 Park Blvd., Ste. 200, Boise, ID 83712 United States
AU: Isaak, D J
EM: disaak@fs.fed.us
AF: U.S.D.A. Forest Service, Rocky Mountain Research Station, 316 E. Myrtle St., Boise, ID 83702 United States
AU: Thurow, R F
EM: rthurow@fs.fed.us
AF: U.S.D.A. Forest Service, Rocky Mountain Research Station, 316 E. Myrtle St., Boise, ID 83702 United States
AB: We examined hydraulic controls on the spatial distribution of chinook salmon spawning gravels in the Middle Fork Salmon River, a mountain basin in the Frank Church Wilderness of central Idaho. Reach-scale predictions of substrate size were integrated via a digital elevation model (DEM) to assess the basin-scale distribution of spawning gravels. Channel reaches were delineated between 40' contour intervals within the DEM, and were further subdivided at tributary junctions. Reach-scale predictions of channel competence (size of sediment that can be carried by the channel) were determined from the Shields equation evaluated at bank-full stage and empirically adjusted for local channel roughness and sediment supply (both as functions of bank-full shear stress). We use the bank-full shear stress because channel morphology and sediment size in coarse-grained rivers are commonly adjusted to bank-full flow. Shear stress in the Shields equation was predicted from a depth-slope product, with bank-full depth determined from a basin-specific hydraulic geometry relationship (expressed in terms of drainage area), and channel slope was determined from the DEM. Data for the empirical relationships were derived from field measurements of 121 channel reaches. The final grain-size prediction is a function of drainage area and slope, values readily obtained from the DEM. Field observations of chinook salmon spawning sites within the study area were used to define preferred substrate sizes. Predicted grain sizes were compared to preferred values to determine the basin-scale distribution of suitable spawning gravels. To test the model, predicted spawning-gravel sites were compared to an 8 year record of observed redd locations. Results demonstrate that nearly 90% of the redds (n=5135) occurred within reaches predicted to have suitable substrate sizes, with 98% of those reaches predicted to have a pool-riffle morphology. Moreover, redd densities were roughly 3 times greater in reaches predicted to be suitable. However, redds tended to be clustered in upstream portions of the accessible network, rather than evenly distributed throughout the suitable reaches. This may be due to a variety of factors, including 1)depressed population sizes and preferential selection of the best spawning sites; 2)loss of chinook salmon phenotypes that once spawned in currently underutilized habitats, and fidelity of remaining subpopulations; or 3)constraints imposed by environmental factors other than substrate size (e.g., stream temperature, velocity, scour regime, hyporheic flow, winter icing, etc.). Sources of error in our predictions are largely due to scatter in the correction for channel roughness and sediment supply. Correspondence between observed channel slopes and those predicted from the DEM are reasonably good. Although reach-scale predictions perform well, spawning site selection likely depends on subreach hydraulics and textural patches that are not accounted for in our analysis. Nevertheless, subreach textural variability can be associated with reach-scale channel type, allowing empirical adjustment for the potential subreach area available for spawning.
DE: 0400 Biogeosciences
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
SC: Hydrology [H]
MN: 2003 Fall Meeting