Mapping increases in hyporheic exchange from channel-spanning logjams

Human impacts such as timber harvesting, channel engineering, beaver removal, and urbanization alter the physical and chemical characteristics of streams. These anthropogenic changes have reduced the number of fallen trees and amount of loose wood that form blockages in streams. Logjams increase hydraulic resistance and create hydraulic head gradients along the streambed that drive groundwater-surface water exchange. Here, we quantify changes in hyporheic exchange flow (HEF) due to a channel-spanning logjam using field measurements and numerical modeling in MODFLOW and MT3DMS. Electrical resistivity (ER) imaging was used to monitor the transport of solutes into the hyporheic zone during a series of in-stream tracer tests supplemented by in-stream monitoring. We conducted experiments in two reaches in Lithe Beaver Creek, Colorado (USA): one with a single, channel-spanning logjam and the second at a control reach with no logjams. Our results show that 1) higher HEF occurred at the reach with a logjam, 2) logjams created complex HEF pathways that caused bimodal solute breakthrough behavior downstream, and 3) higher discharge rates associated with spring snowmelt increased the extent and magnitude of HEF. Our numerical modeling supported all three field findings, and also suggested that lower flows increased solute retention in streams, although this last conclusion was not supported by field results. This study represents the first use of ER to explore HEF around a naturally occurring logjam over different stream discharges, and has implications for understanding how logjams influence the transport of solutes, the health of stream ecosystems, and stream restoration and conservation efforts.