PNNL

Abstract

The flow magnitude and timing from hydroelectric dams in the Snake River basin of the Pacific northwestern United States is managed in part for the benefit of salmon. The objective of this research was to evaluate the effects of current Hells Canyon Dam discharge operations on hydrologic exchange flows between the river and riverbed in Snake River fall Chinook salmon spawning areas. Interactions between river water and pore water within the upper 1 m of the riverbed were quantified through the use of self-contained temperature and water level data loggers suspended inside of piezometers. The data were recorded at 20 min intervals over a period of 200 days when the mean daily discharge was 218–605 m3 s–1, with hourly stage changes as large as 1.9 m. Differences in head pressure between the river and riverbed were small, often within ±2 cm. Measured temperature gradients in the riverbed indicated significant interactions between the surface and subsurface water. Neither hydraulic nor temperature gradients at most sites were significantly affected by either short- or long-term changes in discharge operations from Hells Canyon Dam. Only 2 out of 14 study sites exhibited acute flux reversals between the river and riverbed resulting from short-term, large magnitude changes in discharge. The findings suggest small-scale piezometric head differences play a minor role in the hydrologic exchange between the river and riverbed at the study sites. The processes controlling hydrologic exchange at the study sites are likely to be bedform-induced advective pumping, turbulence at the riverbed surface, and large-scale hydraulic gradients along the longitudinal profile of the riverbed. By incorporating the knowledge of hydrologic exchange processes into water management planning, regional agencies will be better prepared to manage the limited water resources among competing priorities that include salmon recovery, flood control, irrigation supply, hydropower production, and recreation.