Electrical imaging of hyporheic exchange from channel-spanning logjams

Abstract

Human impacts such as timber harvesting, engineered channels, beaver removal, and urbanization can alter the inherent 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 hydraulic resistance and create hydraulic head gradients along the streambed that drive groundwater-surface water exchange. This exchange occurs in saturated sediment under and around a stream, called the hyporheic zone, where surface water and groundwater mix. Hyporheic exchange has an important influence on a stream’s ecosystem, because it transfers dissolved oxygen, solutes, and nutrients into the subsurface as well as mediates temperature fluctuations. Here, we focus on quantifying the changes in hyporheic exchange flow (HEF) due to channel-spanning logjams. Field measurements and numerical modeling using MODFLOW and MT3D were used in this study to explore logjam-induced hyporheic exchange. The traditional methods for characterizing HEF, such as in-stream and well monitoring, fail to capture the complex hyporheic processes because they only provide point measurements. Electrical resistivity imaging (ERI), a surface-based geophysical method, was used to monitor the transport of solutes into the hyporheic zone during an in-stream tracer test supplemented by in-stream monitoring. ERI provides spatial and temporal data on the distribution of subsurface bulk electrical resistivity. We ran ERI at two reaches in Little Beaver Creek, CO: one with a single 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 than the one without, and that 2) higher discharge rates associated with spring snowmelt increase the extent and magnitude of HEF, while 3) lower flows may increase the residence time in the hyporheic zone. The numerical modeling in MODFLOW and MT3D supports the finding that logjams increase the extent and rate of HEF. This research has implications for quantifying the controls of natural stream heterogeneity, the transport of sediment, the health of the stream’s ecosystem, and improving stream restoration and conservation efforts.