ROBERT K. FAHNESTOCK AWARD: ELECTRICAL IMAGING OF THE CHANGES IN HYPORHEIC EXCHANGE FROM CHANNEL-SPANNING LOGJAMS

Human impacts such as timber harvesting, engineered channels, beaver removal, and urbanization can alter the natural characteristics and features of streams, which affect their natural physical and chemical states. One such feature substantially diminished by anthropogenic changes is the development of blockages from fallen trees and loose wood in streams. These logjams increase the hydraulic resistance and create pressure gradients along the streambed that drive hyporheic exchange flow. Hyporheic exchange has a significant influence on a stream’s ecosystem, because it transfers dissolved oxygen, solutes, and nutrients into the subsurface as well as mediates temperature fluctuations. Here, field measurements and numerical modeling using MODFLOW and MT3D were used to quantify the changes in hyporheic exchange due to increasing channel complexity relating to channel-spanning logjams.

The traditional methods for characterizing the hyporheic zone such as instream and well measurements fail to capture the complex hyporheic processes, because they only provide point measurements and require substantial assumptions for estimating hyporheic exchange rates. Electrical resistivity imaging (ERI), a surface-based geophysical method, was used to image the transport of solutes into the hyporheic zone during an instream tracer test supplemented by instream monitoring. This provides spatial and temporal data on the distribution of subsurface bulk electrical resistivity. ERI was run in a single channel parallel to logjams as well as at a control section with no logjams. Our results show that the residence time of the surface water in the hyporheic zone increased as did the amount of solute transferred into the hyporheic zone in the presence of logjams. This research has implications for quantifying the controls of natural heterogeneity in a stream channel, the transport of sediment, the health of the stream’s ecosystem, and improving stream restoration and conservation.