Introduction
Reintroducing gravel to rivers is emerging as a new approach to river restoration when dams and reservoirs disrupt the natural downstream transport of sediment. Although gravel augmentation is primarily used to allow rivers to rebuild bars, riffles, and other habitat features, it may also help mitigate thermal impacts by increasing hyporheic exchange. Hyporheic exchange occurs when surface water enters the riverbed and flows along subsurface paths before returning to the main channel. This exchange removes heat/water from the channel when temperature/discharge is high and releases heat/water to the channel when temperature/discharge is low. Therefore, hyporheic exchange is a dominant mechanism for buffering stream temperature, and may dampen daily or seasonal fluctuations, decrease maximum temperatures, and increase minimum temperatures. Hyporheic flow in alluvial rivers like the Clackamas is primarily associated with gravel bedforms, such as bars and side channels. Water enters the bedform, travels through it, and exits at rates determined by the character and length of the hyporheic flow paths, and bed sediment characteristics that determine hydraulic conductivity.
Our field campaign during Summer 2006 characterized the geomorphology and temperature conditions of the current gravel bar network on the 15-mile reach below River Mill Dam on the Clackamas River (Fig. 1). We first performed a reach-scale survey of the distribution, sizes and types of gravel bar on the river, allowing us to identify and characterize representative bar types. Surface water temperatures associated with these bars were also measured in order to identify temperature anomalies, locations where the temperatures are >1°C different from the mainstem temperature. Selected representative bars were then chosen for detailed surveying and grain-size analysis, surface and subsurface temperature mapping, and subsurface measurement of hyporheic flow. In addition, Thermal Infrared Imagery (TIR) provided further verification of temperature anomalies at the reach scale and their relationship to bar geomorphology.
With these measurements, we are able to assess three factors critical to estimating the impact of gravel augmentation on stream temperature: (1) the timescales of hyporheic exchange; (2) the volume of hyporheic zone; (3) the locations on the river where hyporheic exchange is occurring. By understanding the current magnitude and types of hyporheic exchange on the Clackamas River, we are then capable of making predictions of the possible impact of gravel augmentation on temperature.