The influence of in-stream structures on summer water temperatures via induced hyporheic exchange

Abstract

Temperature is an important controlling factor for ecological functions. In-stream geomorphic structures affect stream thermal regimes by facilitating hyporheic exchange of water and heat between stream channels and underlying sediments. We varied the height of an experimental weir (representing debris dams, log dams, and boulder weirs) in a small stream during the summer and monitored the hydraulic and thermal response of surface and subsurface water using a three-dimensional sensor array. The presence of the structure altered stream temperature patterns, increasing thermal heterogeneity in surface water and shallow sediments by up to ∼1.0°C. We estimated heat conduction and weir-induced hyporheic heat advection across the streambed, and evaluated their response to key parameters. Conduction and advection were of similar magnitude and oscillated over the stream's diel temperature cycle. Weir-induced hyporheic heat advection caused slight cooling of the surface stream (up to ∼0.01°C), and increased with weir height, but was considerably less important to the overall heat budget of the stream than was atmospheric heat exchange. Streambed hydraulic conductivity appears to be the overriding factor determining the magnitude of weir-induced hyporheic influence on surface water temperatures. We conclude that weir-type structures will induce ecologically significant surface and subsurface thermal heterogeneity in many stream settings, but that weir-induced hyporheic heat advection will have ecologically significant thermal effects on surface water only in coarse streambeds. Because these structures are common in natural streams and stream restoration projects, such thermal effects may be important on a landscape level. © 2009, by the American Society of Limnology and Oceanography, Inc.