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

This research evaluated relationships among river discharge, hyporheic zone characteristics, and egg pocket water temperature in Snake River fall Chinook salmon spawning areas. This was a pilot-scale study to evaluate these relationships under existing operations of Hells Canyon Dam (i.e., without any prescribed manipu¬lations of river discharge) during the 2002–2003 water year. The project was initiated in the context of examining the potential for improving juvenile Snake River fall Chinook salmon survival by modifying the discharge operations of Hells Canyon Dam. 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 at index sites throughout 160 km of the Hells Canyon Reach (HCR) of the Snake River. Interactions between river water and pore water within the riverbed (i.e., hyporheic zone) at each site were quantified through the use of self-contained temperature and water level data loggers suspended inside of piezometers. Surrounding the piezometer cluster at each site were 3 artificial egg pockets. The effects of discharge on vertical hydrologic exchange between the river and riverbed were inferred from measured temperature gradients between the river and riverbed, and the application of a numerical model.


  • Hanrahan, T. P., D. R. Geist, E. V. Arntzen, and C. S. Abernethy. 2004. Effects of hyporheic exchange flows on egg pocket water temperature in Snake River fall chinook salmon spawning areas. Final report to Bonneville Power Administration. PNNL-14850.


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