| Summary: | Dynamic exchange of water across the stream-riparian zone interface is important in increasing stream water transit time through basins and enhancing redox-sensitive biogeochemical reactions that influence downstream water quality and ecosystem health. Such exchange may be enhanced by beaver dams, which are common throughout low order streams in North America, Europe, and Argentina. Lateral exchanges of water and nitrogen (N) were observed along a beaver dammed, third-order stream draining a 1.3 km2 Canadian Rocky Mountain valley bottom capped in peat. Measurements of hydraulic heads and chloride concentrations from a network of 80 water table wells were used to identify areas of stream water and groundwater mixing in the riparian area, and their spatiotemporal dynamics in summer 2008. Beaver were found to be the greatest factor affecting lateral movement of channel water into the riparian area. Channel water flowed laterally into the riparian area upstream of the dams and back to the channel downstream of the dams. The hyporheic zone expanded by ¡Ü1.5 m in the un-dammed reaches, but upwards of 7.5 m or more when dams were present. High contributions of stream water were found far out in the riparian area where dams were not immediately present within the stream reach, suggesting that upstream dams directed stream water into the riparian area where it travelled down valley before returning to the stream. This suggests that multiple dams create hyporheic flow paths at multiple scales. Potential mass flux calculations show the riparian area immediately downstream of the beaver dam was a source of N and dissolved organic carbon (DOC) to the stream, and a sink along the rest of the reach. Cold spots of N and DOC availability were also found along the beaver-driven flow paths in the riparian area adjacent to the dam. This pattern likely developed due to flushing of nutrients along the beaver driven hyporheic flow vectors. This work enhances our understanding of stream-aquifer exchange and N dynamics in riparian areas, and the effects of beaver on these processes.
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