USGS Water Data for the Nation
Real time river flow, temperature and conductivity for the Potomac River at Little Falls
Water quality for Virginia's tributaries of the Potomac River
EPA's "Surf Your Watershed" or go directly to one of the following watershed pages:
Middle Potomac-Anacostia-Occoquan Watershed -- Middle Potomac-Catoctin Watershed
LiveBinder full of Map Resources, Learning Games, Stewardship Project Ideas...
Fairfax County
Burke Lake, Lake Accotink and/or Lake Fairfax