1D SIMULATION OF HYPORHEIC EXCHANGE IN A STREAM DAMMED BY BEAVER

Hyporheic exchange, the exchange of water between streams and adjacent subsurface sediments, is an important interface between groundwater and surface water. Knowing the location and magnitude of hyporheic exchange is useful in evaluating the fish spawning habitats, biogeochemical processes, and capacity for aquifer recharge of a given stream. Hyporheic exchange through the streambed is driven by variations in longitudinal head gradient. Modifications to bed topography, such salmon nests, boulders, fallen logs, and beaver dams, all promote increased hyporheic exchange. The magnitude of exchange typically increases as more of the stream flow is obstructed. Beaver dams are unique in that they are designed to be channel-spanning obstructions at least as tall as the stream depth, often obstructing flow so effectively that a large pond forms upstream of the dam. Beaver dams further change the upstream topography within their ponds through increased sedimentation rates and excavation of pond-bottom mud by the beavers. It follows that the dominant drivers of hyporheic exchange in beaver-occupied streams will be the beaver dams.

This 1D model of hyporheic exchange is based on Laplace’s Equation and Darcy’s Equation. Each simulation assumes steady-state conditions and isotropic, homogenous hydraulic conductivity. I utilize both observed and theoretical streambed profiles in streams dammed by beaver to explore how the hyporheic exchange in a stream might change through time as a beaver dam increases in height to achieve an ideal pond depth, sediment aggrades, and sediment is excavated by the beavers. The simulations demonstrate that on relatively short timescales (months-years) beaver damming can lead to more pronounced, highly heterogeneous zones of hyporheic exchange when compared to both undammed streams and streams with smaller scale flow obstructions. This result is consistent with previous field and laboratory studies in the literature.