USGS (main site, includes links to NED Hydrologic Flow Derivative Data)
USGS (Water Resources, Real-time and Historic Streamflow)
USGS H2O (Download HSPF from this site)
USGS (Download MODFLOW-96 from this site)
Hydrologic Engineering Center (US
Army Corps of Engs, download HEC programs)
CDEC (California Data Exchange Center)
U. S. General Soil Map (STATSGO2)
Hydrocomp, Inc.