Effects of Hyporheic Exchange Flows on Egg Pocket Water Temperature in Snake River Fall Chinook Salmon Spawning Areas

Abstract

The development of the Snake River hydroelectric system has affected fall chinook salmon smolts by shifting their migration timing to a period when downstream reservoir conditions are unfavorable for survival. Subsequent to the Snake River chinook salmon fall-run Evolutionary Significant Unit being listed as Threatened under the Endangered Species Act, recovery planning has included changes in hydrosystem operations to improve water temperature and flow conditions during the juvenile chinook salmon summer migration period. In light of the limited water supplies from the Dworshak reservoir for summer flow augmentation, and the associated uncertainties regarding benefits to migrating fall chinook salmon smolts, additional approaches for improved smolt survival need to be evaluated. This report describes research conducted by PNNL that evaluated relationships among river discharge, hyporheic zone characteristics, and egg pocket water temperature in Snake River fall chinook salmon spawning areas. The potential for improved survival would be gained by increasing the rate at which early life history events proceed (i.e., incubation and emergence), thereby allowing smolts to migrate through downstream reservoirs during early- to mid-summer when river conditions are more favorable for survival. PNNL implemented this research project throughout 160 km of the Hells Canyon Reach (HCR) of the Snake River.more » The hydrologic regime during the 2002?2003 sampling period exhibited one of the lowest, most stable daily discharge patterns of any of the previous 12 water years. The vertical hydraulic gradients (VHG) between the river and the riverbed suggested the potential for predominantly small magnitude vertical exchange. The VHG also showed little relationship to changes in river discharge at most sites. Despite the relatively small vertical hydraulic gradients at most sites, the results from the numerical modeling of riverbed pore water velocity and hyporheic zone temperatures suggested that there was significant vertical hydrologic exchange during all time periods. The combined results of temperature monitoring and numerical modeling indicate that only two sites were significantly affected by short-term (hourly to daily) large magnitude changes in discharge. Although the two sites exhibited acute flux reversals between river water and hyporheic water resulting from short-term large magnitude changes in discharge, these flux reversals had minimal effect on emergence timing estimates. Indeed, the emergence timing estimates at all sites was largely unaffected by the changes in river stage resulting from hydropower operations at Hells Canyon Dam. Our results indicate that the range of emergence timing estimates due to differences among the eggs from different females can be as large as or larger than the emergence timing estimates due to site differences (i.e., bed temperatures within and among sites). We conclude that during the 2002-2003 fall chinook salmon incubation period, hydropower operations of Hells Canyon Dam had an insignificant effect on fry emergence timing at the study sites. It appears that short-term (i.e., hourly to daily) manipulations of discharge from the Hells Canyon Complex during the incubation period would not substantially alter egg pocket incubation temperatures, and thus would not affect fry emergence timing at the study sites. However, the use of hydropower operational manipulations at the Hells Canyon Complex to accelerate egg incubation and fry emergence should not be ruled out on the basis of only one water year's worth of study. Further investigation of the incubation environment of Snake River fall chinook salmon is warranted based on the complexity of hyporheic zone characteristics and the variability of surface/subsurface interactions among dry, normal, and wet water years.« less