The Effect of Beaver Dams on Geochemistry of the Hyporheic Zone at Varied Depth and Location over a Range of Discharges During Flood Recession

As beaver populations increase in the Western United States, beaver dams are becoming more numerous in many incised low-order streams. These dams raise the water table locally, enhancing riparian zone connection and creating pools that increase stream water residence time. Additionally, the stepped drop in stream head over dams enhances hyporheic exchange and creates spatially and temporally transient seepage flux patterns, which may be sensitive to stream discharge. As discharge falls during flood recession, the two main drivers of hyporheic exchange, head and velocity, should also change. Patterns of high seepage flux into the hyporheic zone, indicated by high dissolved oxygen and concentrations of redox sensitive species (i.e. NO3-, Fe, SO42-), may then be transient during periods of large changes in stream discharge. Cherry Creek, a 2nd order stream outside of Lander Wyoming, has a ~2 km reach populated with beaver dams. A small beaver dam (~35 cm in height) and large dam (~75 cm in height) were instrumented with nested piezometers equipped with sampling ports screened at 5-10, 15-20, 30-35, and 50-55 cm intervals. Four sampling stations were installed above the small dam and five above the large dam in locations chosen to best encompass the local geomorphic complexity. Samples were drawn four times at each location over the summer when stream discharge was 383L/s, 331L/s ,268L/s and 259 L/s to determine the effect of changing stream discharge on flux patterns and geochemical conditions in the hyporheic zone. To analyze these flux patterns, pH and DO were measured on-site at the time of sampling and cation (Mg2+, Ca2+, Na+, NH4+) and anion (SO42+, Cl-, NO3-) concentrations were measured in the laboratory using ion chromatography. These data were collected in conjunction with heat flux measurements using distributed temperature sensing (DTS) to determine hyporheic flux patterns. Decreases in dissolved oxygen concentrations in many locations were observed as the stream discharge and stage declined, indicating less hyporheic mixing during times of low flow. In addition, variable redox patterns, as indicated by covarying SO42+ and NO3- concentrations, were observed with depth in the streambed, as well as between the various geomorphic conditions at each of the piezometer nests. Generally, streambed geochemical profiles at high seepage flux zones, such as gravel bars in close proximity to the dam, most closely resembled stream water. In contrast, the geochemical profiles beneath pools and gravel bars further from the dams have lower DO and low concentrations of redox sensitive species, such as SO42+ and NO3-. The differences between the geochemical profiles at varied discharge rates and at individual locations demonstrate the complicated transient effects that beaver dams have on hyporheic interaction within this system.