Interacting Influence of Log Jams and Branching Channels on Stream-Groundwater Exchange

Log jams restructure the hyporheic zone, or region where stream water and groundwater mix, by storing sediment, widening the stream in backwater areas, forcing new channel branches, and altering hydraulic gradients that drive hyporheic exchange. Here, we use flume and numerical experiments to quantify the effects of interacting jam structures and channel branches on hyporheic exchange at three stream flow rates. The presence of multiple jams increased wetted streambed area (the area available for hyporheic exchange) by 9-38% and increased hyporheic fluxes across the bed by roughly an order of magnitude, leading to an order-of-magnitude decrease in the turnover length that stream water travels before interacting with the hyporheic zone. Decreased turnover lengths corresponded with greater reaction significance per km, a measure of the potential for the hyporheic zone to influence stream water chemistry. For low-flow conditions, log jams increased reaction significance per km five-fold, from 0.07 to 0.35. Jams with larger volumes led to longer hyporheic residence times and path lengths that exhibited multiple scales of exchange. Additionally, the longest flow paths connecting multiple jams occurred in the reach with multiple channel branches. These findings suggest that large gains in hydrologic connectivity can be achieved by promoting in-stream wood accumulation and the natural formation of both jams and branching channels. More studies are needed at field scales to understand relationships between jams, wetted channel area, and hyporheic fluxes under natural and more complex conditions.