Hyporheic Cooling with Implications for Support of Fish Habitat in the Willamette Valley, Oregon

It has been hypothesized that groundwater flow which originates from a river and then returns to it could result in a temperature buffering benefit, resulting from dissipation of heat during porous media flow. We installed 50 monitoring wells in a geomorphologically active area along the Willamette River in order to characterize wet and dry season hyporheic flow patterns. The wells were instrumented with pressure transducers with temperature logging, so that we could calibrate groundwater flow models. The sampling network was dense enough so that we could monitor the temperature and stable isotopic variation along individual groundwater pathlines. In one case we observed a temperature decrease of over 6 degrees Celsius along a distance of about 600 m, over a period of about 3 years. In observations along other pathlines we observed considerably lower temperature decreases, but still notable. This presentation will examine some of the possible mechanisms for the temperature decreases and the possible effect of seasonal shifts in flow vectors along hyporheic pathlines.