The Potomac River begins as a small spring near Fairfax Stone, West Virginia. Like a giant funnel it gathers water from Pennsylvania, West Virginia, Maryland, Virginia, and the District of Columbia as it travels 383 miles to the Chesapeake Bay. When it reaches Great Falls, the river narrows and drops 76 feet into a canyon called Mather Gorge. Changing weather conditions in the river's watershed affect the character of the river at Great Falls.