We investigated whether sand seals form in the upper egg pocket of salmonid redds and improve egg survival in a sediment‐impacted coastal stream in northern California. Sand seals can potentially reduce infiltration of detrimental finer sand and silt into the lower egg pocket. We predicted sand seals would form when the redds were exposed to streamflows high enough to entrain coarse sand and form seals. Using artificial redds of coho salmon Oncorhynchus kisutch, we conducted sediment analyses of the upper and lower egg pockets and found that protective sand seals formed in redds when discharge was two or more times the flow that entrains the median particle size of the streambed. When the coarse sand in the upper egg pocket was incorporated into a two‐stage model that predicted survival to hatching and emergence, it greatly improved predictions in years with higher flows in both natural and artificial redds. However, the sand seals provided little protection when suspended sediment flux was high from logging or road construction. We built the model used for these predictions with data from artificial redds and applied it to natural redds during 6 years of different flow regimes. Predictor variables included cumulative flow above the entrainment flow, peak discharge, coarse sand in the upper egg pocket, fine sediment in riffle gravel, coarse sand in the upper and entire egg pocket, suspended sediment flux, and presence of predaceous worms. The model explained 67% of the variance in egg survival to emergence in coho salmon natural redds. We suggest that managers consider the complex interactions of streamflow, sediment transport, formation of sand seals, and fine‐sediment infiltration when estimating salmonid reproductive success in sediment‐impacted streams.
Existing ice and trash sluiceways are commonly used as benign, nonturbine routes for downstream passage of juvenile salmonids at hydropower dams. At The Dalles Dam on the Columbia River, we studied various operational configurations of sluiceway weirs to maximize sluiceway passage of juvenile Pacific salmon Oncorhynchus spp. and steelhead O. mykiss. We applied hydroacoustic methods to compare fish passage rates and sluiceway efficiencies for two weir configurations in each year: three weirs (SL 1; i.e., collectively referring to the three weirs above main turbine unit [MU] 1) versus six weirs (SL 1+18) during 2004; and middle (SL 2+5) versus east (SL 2+19) powerhouse weir locations during 2005. Horizontal distributions at the sluiceway and turbines and the effects of operating turbines beneath open sluiceway gates were also analyzed. Sluiceway passage efficiency relative to the powerhouse (SLYphs) varied between study years, between spring and summer, and between day and night. In 2004, sluiceway passage rates were significantly higher (P = 0.0003) for SL 1+18 than for SL 1 during summer–night but were not significantly different between the two configurations during the other three season–day/night periods. The SLYphs was significantly higher for SL 1+18 than for SL 1. The location comparison during 2005 revealed no significant differences between the SL 2+5 and SL 2+19 configurations, except for summer–day. The experimental findings led to recommendations for long‐term operations of The Dalles Dam sluiceway: open six rather than three sluiceway weirs to take advantage of the maximum hydraulic capacity of the sluiceway; open the three weirs above the westernmost operating MU and the three weirs at SL 8, where turbine passage rates are relatively high; operate the MUs below open sluiceway weirs as a standard procedure; operate the sluiceway 24 h/d year‐round to maximize its benefits to juvenile salmonids; and use the same weir configuration year‐round. These operational concepts are transferable to dams where sluiceway surface flow outlets are used to protect downstream‐migrating fishes.Received July 19, 2012; accepted June 26, 2013
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