The goal of this research is to quantify the role of beaver dams on hyporheic nitrogen cycling by comparing the same reach of a stream under undammed and dammed conditions.  An array of nested streambed samplers and wells adjacent to the stream were sampled monthly for nitrate, ammonium, and quarterly for DON.  A bromide injection test was also performed under dammed conditions.  Bromide was detected in samples 1 m deep in the streambed and <1 m from the stream bank within 8 hours of injection.  This corresponds to residence times along hyporheic flow paths beneath and around the dam on the order of a few days.  Under dammed conditions throughout the fall, the hyporheic flow under and around the dam is a net sink for nitrogen entering from the stream, removing all the nitrate, most of the DON, but adding ammonium.  In January, when the dam was partially degraded, hyporheic flow continued to be a net sink for nitrogen, removing nitrate and DON and adding ammonium.  Numerical modeling from a subsurface perspective is currently being conducted to quantify the flux of water and nitrogen across the sediment-water boundary.