If you want to cite the program in general, use:


U.S. Geological Survey, 2016, The StreamStats program, online at http://streamstats.usgs.gov, accessed on (give date of access).


If you want to cite a specific state application, for example, use:


U.S. Geological Survey, 2016, The StreamStats program for Colorado, online at http://water.usgs.gov/osw/streamstats/colorado.html, accessed on (give date of access). Change the state name (all lower case in the URL) to the name of another state to cite StreamStats for that state.