Engineered Gravel Trench Hyporheic Exchange to Create Cold‐Water Thermal Refuges

ABSTRACT Warming rivers are driving a loss or fragmentation of cold‐water habitat and providing the impetus to develop proactive thermal management approaches to maintain suitable habitat in rivers. One innovative approach is through the creation of cold‐water thermal refuges during periods of thermal stress for aquatic species. In this study, we developed two engineered hyporheic zones consisting of gravel‐filled trenches constructed across natural channel meanders in a river with low‐permeability streambed and banks. Peak outlet water temperatures in system 1 were lowered by 2.2°C and lagged 3.5 h relative to the mainstem river temperature. In system 2, peak water temperatures were lowered 3.5°C and lagged 7.5 h. Fish were visually observed aggregating near the outlet of trench system 2, possibly due to the increased thermal contrast between the discharged water and ambient river conditions. One limitation of such engineered hyporheic zones is that there is a tradeoff between slow‐flowing (low‐permeability) trench systems that lower hyporheic zone temperatures through long residence times but discharge too slowly to create a large cold‐water plume, and fast‐flowing systems that are dominated by lateral heat advection from the river and consequently minimal reductions in temperature. Thermal numerical modeling was conducted to interpret the field data and assess the impact of alternative designs. Model results suggest that increasing the trench length (increased residence time) decreases the daily mean and peak outlet water temperatures compared to river water temperatures. The study findings demonstrate challenges and opportunities for thermal management approaches aimed at maintaining thermal diversity in warming rivers.