Log jams are trees that frequently fall and accumulate in rivers. Field surveys and numerical simulations suggest that log jams slow down the surface flow and drive hyporheic flows, which are bidirectional flows that go into the riverbed and back to the surface water. Hyporheic flows carry pollutants and nutrients and thus play a critical role in water quality and river biogeochemical cycles. Despite the importance of hyporheic flows, the quantitative characterization of log jam-induced hyporheic flows remains incomplete. 

In this study, we conducted experiments in a water-recirculating flume with a log jam model that resembles piles of wood logs commonly found in rivers. We injected a fluorescent dye into a transparent sediment bed made from hydrogel beads and visualized the flow within the sediment bed. Our experimental results show that log jams can increase the hyporheic flow rate by one order of magnitude. Further, we developed a theoretical model to explain the hyporheic flow induced by a log jam. Our experimental results and theoretical model will facilitate the evaluation of the impact of log jams on the fate and transport of nutrients and contaminants in rivers for future restoration projects.