Morphology-Induced Thermal Refuge in a Gravel-Bed River

Abstract

Climate change is increasingly impacting stream temperature, a primary control on the biological, geochemical and ecological processes in fluvial systems. In response, river restoration is focusing on river temperature regimes and creating instream thermal refuge during peak summer temperatures. Here, we evaluate the effectiveness of various constructed morphologic features (e.g., pools, riffles, alcoves, plane beds and spring-fed side channels) designed to generate thermal refuge during summer low-flow conditions in a recently restored gravel-bed river. To assess this, we monitored the groundwater table, river water surface elevation, the direction and magnitude of hyporheic fluxes and the spatial distribution of near-bed water temperatures. River-groundwater differentials, quantified as the difference in elevation between the surface water elevation in the river and the groundwater table, were the primary factors controlling hydrologic gaining and losing patterns in the channel. In contrast, flow-bedform interactions generated by individual morphologic features induced comparatively limited hyporheic exchange, with average hyporheic fluxes constituting ~0.25% of the instream summer discharge. While the magnitude of hyporheic fluxes was relatively similar across individual morphologic features, pools contained the most thermal refuge and longitudinally cooled water temperatures by an average of 1.26°C/100 m. In contrast, riffle and plane bed features generated little observed thermal refuge and increased temperatures on average by 1.43°C/100 m and 0.81°C/100 m, respectively. Spring channels provided cool water temperatures at their upstream ends, but the slow, shallow flows rapidly warmed (3.7°C/100 m) before entering the main river channel due to lack of riparian shade. Alcove temperatures were similarly influenced by depth and riparian shading. Deep, well-shaded alcoves provided cool-water habitat, whereas shallow, unshaded features maintained high temperatures. Results outline the role of channel morphology in generating thermal refuge that may be used to better understand aquatic habitat and guide future restoration projects in gravel-bed rivers.