The influence of Castor canadensis (North American Beaver) impoundments on the loss of nitrate (NO₃^-) from stream water was monitored for 14 months at four sites in Homer Gulf Creek in central New York State. A relatively high NO₃^- concentration of 1.56 (0.14) mg N/L (mean (SEM)) at the upstream site was reduced by a daily average of 35.5% as the water passed through the beaver ponds. There was a significant difference (P < 0.001) in the mean values among the four sites, and significant differences were measured between the upstream site and each of the three downstream sites. The reduction in NO₃^- within the upstream beaver pond was significantly (P = 0.005) greater during the warmer months (April–September) of the study vs. the October–March period, suggesting biological processes were responsible. Stream water NO₃^- may have been transformed through biological processes, including the microbial process of denitrification, enhanced in the beaver ponds as a result of anoxic sediments, sufficient supplies of labile organic matter, and increased water residence time.