SEDIMENT DISTRIBUTION AROUND DEBRIS DAMS: IMPACTS ON STREAMBED HYDROLOGY, BIOGEOCHEMISTRY AND TEMPERATURE DYNAMICS IN SMALL STREAMS

Debris dams increase stream channel complexity, slow stream water velocity, and divert water into the subsurface, thereby enhancing stream-groundwater interactions. Sudden drops in stream stage around debris dams generate steep head gradients in the streambed. Slow-moving water behind dams allows fine sediments to settle out and accumulate in upstream pools. Downstream of debris dams, stream water is starved of fine-sediment and rapid flow creates streambed zones with high hydraulic conductivity (K). We have used field experiments and numerical modeling to evaluate the impact of debris dams on hyporheic exchange in Red Canyon Creek, a semi-arid stream in Wyoming, USA. To capture spatial and seasonal variability of stream-groundwater interactions, we observed the hydraulic gradient, diurnal temperatures and pore water geochemistry at 30 points arranged in 1 meter by 1 meter grids upstream and downstream of debris dams during October, 2005, and July, 2006. To capture temporal variability, we observed the diurnal and seasonal temperatures in longitudinal profile along the thalweg upstream and downstream of a dam. We have also used MODFLOW and MT3D to simulate the flow dynamics around dams during steady-state baseflow conditions.

Debris dams create hyporheic zones that are volumetrically larger than hyporheic zones around other geomorphic features. The complex head gradients and sediment distributions around debris dams cause hydrologic flow paths and biogeochemical and thermal dynamics to be highly variable in space. Upstream of debris dams, strong downward head gradients and low K values create low-velocity flowpaths from the stream to the streambed with steep redox gradients. Here, diurnal and seasonal temperature dynamics are less variable and more indicative of the groundwater thermal regime. Streambed pore waters upstream of dams indicate significant iron, manganese and sulfate reduction. Downstream of debris dams, head gradients are small and variable and K values are high, generating rapid surface-groundwater exchange. Here, diurnal and seasonal temperatures vary significantly and are more indicative of the stream's thermal regime. Pore waters in the streambed are highly oxygenated, similar in geochemical composition to the stream water and show evidence of nitrate production.