Mapping Increases in Hyporheic Exchange from Channel-Spanning Logjams

Doughty, Megan, Colorado School of Mines, Army Corps of Engineers, megan.jean.colorado@gmail.com; Audrey Sawyer, sawyer.143@osu.edu; Ellen Wohl, Ellen.Wohl@colostate.edu; Kamini Singha, ksingha@mines.edu (Poster)

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 Little Beaver Creek, Colorado: 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) log- jams create complex HEF pathways that can cause bimodal solute breakthrough behavior downstream, and 3) higher discharge rates associated with spring snowmelt increase the extent and magnitude of HEF. The numerical modeling supports all three field findings, and also suggest that lower flows increase solute retention in streams, although this last conclusion is 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 under- standing how logjams influence the transport of solutes, the health of stream ecosystems, and stream restoration and conservation efforts.

Bio:

Megan Doughty is currently a Civil Engineer at Risk Management Center with Army Corps of Engineer. She works with hydrology and hydraulics group to model surface water conditions and analyze flood risks. She received her Bachelor’s degree in Geological Engineering and her Master’s in Hydrology from the Colorado School of Mines. The research presented here is from her Master’s research and was published in the Journal of Hydrology at the beginning of 2020. 

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