Imaging the Subsurface: The Effect of Logjams on Groundwater-Surface Water Exchange

Abstract

Conventional methods of quantifying subsurface interactions in streams are often insufficient in characterizing hydrological processes and demand better techniques to measure complex dynamics. Point-measurements are incomplete and do not provide direct data on hyporheic exchange, a mixing of surface and subsurface water under and around a stream capable of influencing ecosystem processes. Fortunately, geophysical techniques can improve analysis of the hyporheic zone. For the first time in a field setting, we apply electrical resistivity imaging (ERI) to a mountainous stream below a channel-spanning logjam to estimate the extent of hyporheic exchange. Previous studies suggest logjams increase hydraulic resistance and drive water deeper into the hyporheic zone; our results imply water remains in the subsurface longer around a logjam, thus increasing a stream’s ability to process nutrients and solutes. A comparison between fluid conductivity and bulk conductivity proved ERI as a more spatially comprehensive technique in quantifying the extent of hyporheic exchange. These results highlight the critical role that logjams play in hyporheic zone dynamics, which could affect ecosystem health: future applications of this study can aid in conserving, managing, and restoring riverine systems.