Reclamation Information Sharing Environment (RISE)

River restoration targeted at promoting thermal refuge for cold-water salmonids is often critical for encouraging fish recruitment and success in channels that are too warm for fish survival. One method of promoting thermal refuge is to increase the exchange of surface-subsurface flow (hyporheic flow) through creating geomorphic diversity. Previous studies have not explored the impacts of these types of features on reach-scale thermal buffering. This study combines field work, two-dimensional hydraulic modeling, and three-dimensional groundwater modeling to address the question of if river restoration can increase hyporheic exchange at the Bird Track Springs restoration site of the Grande Ronde River, Oregon. The model results showed that restoration did increase the overall hyporheic exchange in the channel. However, the magnitudes of upwelling and downwelling were relatively small due to the low hydraulic conductivity of the alluvial material. In spite of this, the reach showed an overall reduction in the maximum summer water surface temperatures and thermal buffering that reduced the temperature variations from 12 °C to 4.5 °C between pre- and post-restoration. Pool stratification can account for the observed reduction in temperatures and thermal buffering. These effects are flow-dependent, and only come into play at very low flows. Field observations showed that thermal refuge was localized to morphologic features that could retain the low magnitude hyporheic discharges.