Rockymarsh Run watershed straddles the northern boundary between Berkeley and Jefferson counties of West Virginia.  The watershed is tributary to the Potomac River, which flows into the Chesapeake Bay.  The 16.9-square mile surface drainage basin of Rockymarsh Run is long and narrow, about 9 miles long and 2 miles wide.  The waters of Rockymarsh Run are cold owing to the multitude of its springs and were once home to our native brook trout as well as many marshes.

There are five major springs in the watershed with year-round flows greater than 100 gallons per minute (gpm).  Three of these have flows greater than 1,000 gpm.  These are Rock Spring, Spring Hill Spring, and Southwood Spring.  Southwood Spring forms the source of the West Fork while Rock Spring forms the source of perennial flow on the East Fork.  There are at least seventeen other springs in the watershed ranging from small year-round flows, seasonal flows, and springs that only flow during extreme wet weather.