HR: 13:30h
AN: NB23F-01 INVITED     [Abstracts]
TI: Coupling Hydrodynamic Modeling and Empirical Measures of Bed Mobility: Implications for Restoring Spawning Gravel Quality on a Large Regulated River
AU: * May, C L
EM: cmay@seismo.berkeley.edu
AF: University of California, Dept. of Earth and Planetary Science, Berkeley, CA 94720-4767 United States
AU: Smith, B J
EM: for_bonnie@yahoo.com
AF: University of British Columbia, Department of Geography, Vancouver, BC V6T 1Z2 Canada
AU: Lisle, T E
EM: tel7001@axe.humboldt.edu
AF: U.S.F.S. Pacific Southwest Research Station, 1700 Bayview Dr., Arcata, CA 95521 United States
AU: Lang, M M
EM: mml1@humboldt.edu
AF: Humboldt State University, Environmental Resource Engineering, Arcata, CA 95521 United States
AB: Flow releases are increasingly being used as a tool to restore spawning gravel quality downstream of large dams. Often times, the primary goal of a peak flow release is to flush fine sediment from incubation habitat and restore active river processes; thus understanding the portion of the bed that is entrained and the flow required for full mobility is important. However, a critical knowledge gap for implementing these experimental floods is predicting the potential scouring of spawning redds in downstream reaches. To address these questions we need to understand the relationships between river discharge, bed mobility, and scour depths in areas heavily utilized by spawning salmon. Our approach couples numerical flow modeling and empirical data to quantify spatially explicit zones of differential bed mobility and identify specific areas where scour is deep enough to impact redd viability. Boundary shear stress values were predicted using the USGS's Multi-Dimensional Surface Water Modeling System for a segment of the Trinity River below Lewiston Dam. From model-generated shear stress and fine-scale mapping of local particle size distributions, Shields stress values were calculated to identify areas of differential bed mobility. Our data suggest that full mobility is limited to a central, yet discontinuous core along the thalweg, which expands with increasing flow strengths. Spatial analysis revealed that Chinook salmon tend not to spawn in areas that became fully mobile during bankfull flood events. Scour depths in areas preferentially used for spawning were less than in other portions of the bed and were not deep enough to impact incubating eggs or embryos. This site-selection preference allows fish to spawn in areas that are relatively safe from deep scour. However, the trade-off for spawning in stable areas is that flushing of fine sediment from the subsurface occurs very infrequently and gravel permeability is likely to become limiting for egg and embryo survival. Extremely large floods will be required to flush fine sediment stored in the subsurface from areas preferentially used for spawning. Preliminary evidence also suggests that the increase in gravel quality associated with the flushing of fines from the subsurface (by mobilization of the bed during high flow events or by fish during the excavation of redds) is short lived because of the large influx of fine bed material from tributaries. This study indicates that a combined strategy of reducing fine sediment inputs and providing peak flows large enough to mobilize substantial portions of the bed but small enough not to scour incubating embryos is needed to maintain favorable spawning and incubation habitat below reservoirs. The optimum range of peak flows can be designed with flow models that predict areas of mobility in relation to areas selectively used by spawning fish.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
SC: North American Benthological Society [NB]
MN: 2005 Joint Assembly