Multidimensional, Reach-Scale Models of Hyporheic Exchange due to Complex Log Jams in Mountain Streams

Large wood is a natural feature in mountain streams that accumulates through treefall, moves by flooding or beaver activity, and forms log jams. Jams promote stream-groundwater interactions, or hyporheic exchange, by influencing gradients in hydraulic head, forming anabranching channels, and increasing alluvial cover in streambeds. We simulate these effects of log jams on hyporheic exchange at Little Beaver Creek, a mountain stream located in the Front Range of Colorado, using fully-coupled three-dimensional surface water-groundwater flow models in HydroGeoSphere. The model reach was based on a publicly-available, LiDAR-generated digital elevation model and supplemented by physical surveys of the channel and jams. Model runs were conducted with and without jam structures for three stream discharges: baseflow (0.11 m³/s), medium flow (0.25 m³/s), and peak flow (1.08 m³/s). When jams are present, the median hyporheic exchange flux is 0.7 to 3 orders of magnitude greater for the three discharges simulated. Jams also create longer flow paths, with median hyporheic flow lengths of 9 to 15 meters, compared to lengths of 1 to 2 meters in the absence of jams. In the presence of jams, median hyporheic residence time decreases with increasing stream discharge, while the opposite occurs without jams. Overall, the hydraulic gradients created by pooling in the vicinity of jams increases the physical and geochemical connectivity between the surface and subsurface in mountain streams.