USGS Water Data for the Nation
The following is an example of how to cite this website in a publication.
U.S. Geological Survey, 2001, National Water Information System data available on the World Wide Web (Water Data for the Nation), accessed [June 10, 2001], at URL [http://waterdata.usgs.gov/nwis/].
You would need to substitute the applicable items enclosed in brackets (remove the brackets too) for the date you accessed the data and URL you are referring to within Water Data for the Nation.
While the development team will take considerable effort to maintain the integrity of existing URLs, queries for specific data within Water Data for the Nation may not function in the future if the software or database is substantially changed.