Automated hydrograph separation techniques were used to evaluate the groundwater contribution to total streamflow (baseflow) at active and inactive stream gauging stations throughout Washington State. Discharge records for 582 gauging stations, with at least three complete water years of daily mean streamflow data, were downloaded from the U.S. Geological Survey (USGS) National Water Information System. Station characteristics were compiled for each gauge, including period of streamflow record, type and degree of regulation affecting the gauge, watershed drainage area, USGS station number, station name, and gauge location. Summary statistics were calculated for annual mean streamflow and annual 7-day low flow for all 582 stations. Monthly, and in some cases annual, statistics for baseflow were then estimated using a USGS hydrograph separation software program called HYSEP (Sloto and Crouse, 1996) for those stations judged to be free of significant snowmelt or regulation effects.