Abstract
Water temperature is a fundamental driver of physical processes, metabolic rates, and habitat availability in fluvial systems. As anthropogenic activities and climate change increase river temperatures and associated thermal stress on aquatic organisms, river restoration has focused on moderating thermal regimes and creating localized cold-water refuges. Restoration of a 2.5 km-long gravel-bed reach of the Grande Ronde River (Oregon, USA) provided a unique opportunity to compare the pre- and post-restoration influences of hyporheic and groundwater discharge and pool stratification in generating local and reach-scale thermal buffering under similar valley and riparian conditions. Field data and numerical simulations demonstrate that hyporheic discharge had limited influence on the surface water temperature because of its low magnitude relative to the streamflow. In contrast, pools excavated during restoration, with flow depths up to 2.2 m, showed thermal stratification during summer low-flow conditions that generated a temperature differential in the pool bottoms up to 4.6 °C. The duration that an individual pool could maintain water temperatures below the mortality threshold for salmonids (T < 25 °C) increased with dimensionless residual pool depth (Dr⁎ = Dr/Df, where Dr is the residual pool depth and Df the mean flow depth). However, no pools were capable of maintaining sublethal temperatures for the entire summer period. The cumulative effect of stratified pools generated reach-scale thermal buffering by exporting cool water downstream during the day. Measured field values indicate that relative to the upstream reach boundary, maximum daily water temperatures at the downstream boundary of the study site would increase by an average of 1.5 °C before restoration but decrease by an average of 3 °C after restoration. At the reach scale, a sequence of stratified pools induced downstream thermal buffering but could not completely relieve the unsuitable warm water temperatures entering the upstream end of the reach.