Changes in Hyporheic Exchange Flow Following Experimental Woody Debris Removal Determined by Groundwater Flow Modeling

Hyporheic exchange flow [HEF] in small streams is strongly influenced by woody debris and geomorphology [e.g., Kasahara and Wondzell, 2002], both of which may be changed naturally or anthropogenically over short time. Despite the clear relation between channel morphology and patterns of HEF, the influence of wood removal on HEF has not been studied. We investigated the response of HEF to woody debris removal in a small, low-gradient, gravel bed stream in Southeast Alaska [Smith et al., 1993] using a series of groundwater models built to simulate hyporheic exchange flows for various stages of the geomorphic response. Seven ground surveys spanning four years post debris removal were used to define the model topography and surveyed water surface elevations were used to assign boundary conditions. Slug tests and salt tracer tests conducted in well networks established in a near-stream gravel bar and adjacent floodplain were used to parameterize hydraulic conductivity and porosity. Using the groundwater flow model, we calculate the flow between the model and the boundary, which is the analog to HEF in the stream. The changes in flow across the boundary of the models relative to pre-removal conditions are considered an effect of the changing geomorphic template driving HEF. We present results comparing whole stream changes in HEF due to woody debris removal and analysis of HEF change at specific locations of geomorphic change such as change in sediment storage, development of thalweg crossovers, and point bar development. While ground-truthed HEF cannot be confirmed, these comparisons improve understanding of the linkages between subsurface hydrology and geomorphic responses to management practices.