We determined the proportion of stream length flowing through beaver-modified habitat using a GIS. We created GIS layers mapping all streams and beaver-modified areas on the HWF in ArcView (ESRI 2000) from digitized, color infrared aerial photographs taken in April 1998. Beaver sites were classified as open water, meadow, or alder sites. We used the map of streams and the map of beaver areas to generate a layer containing only stream segments flowing through beaver-modified areas. To determine the proportion of total stream length in the HWF that flowed through beavermodified habitat, we compared the total length of streams flowing through the beaver-meadow layer to the original stream layer. We had hoped to use maps of regulatory wetlands to determine the percentage of all wetlands on the HWF that are associated with beaver activity. Unfortunately these maps were extremely inaccurate both in placing wetlands in areas where no known wetlands exist, and in mapping the boundaries of known wetlands. As a rough estimate of the percentage of wetlands that are associated with beaver, we first classified all wetlands on the regulatory wetlands map as reliable or not based on visual analysis of aerial photographs and ground truthing. We then took counts of the number of reliable wetlands that overlapped with beaver-modified areas for at least some of their extent.