Broad Run and Horsepen Branch Subwatersheds
The Broad Run originates west of Poolesville near Wasche Road and West Hunter Road. Flowing south toward the Potomac River, it passes through a part of Montgomery County that has changed little in more than 100 years.
This region, a part of the County's agricultural preserve, is characterized by rolling hills, pasture and farmland, and many forested stream buffer areas. The watershed has red sandstone typically found 200-250 million years ago during the Triassic period. The soils tend to be dry; consequently, the Broad Run can have low base flows during the summer months.
When last monitored by County biologists, Broad Run’s stream habitat was in good condition, with stable overhanging banks providing excellent fish cover, frequent riffles, and stream base flow reaching both lower banks.
Twenty-four species of fish were found in the Broad Run, including largemouth bass and three species of sunfish. Water chemistry tests for pH, dissolved oxygen, conductivity, and water temperature were all within state standards.
► Broad Run Watershed Study (PDF, 242KB)
Horsepen Branch originates south of Poolesville near the intersection of Westerly Road and West Willard Road. Like Broad Run, Horsepen Branch is within the red Triassic sandstone geologic area that is unique to this part of the County.
Horsepen Branch flows through the Poolesville Public Golf Course and onto the Izaak Walton League property, where two tributaries join with the mainstem. At River Road, the Horsepen Branch abruptly leaves the upland areas around Poolesville and enters the Potomac River floodplain. Above River Road, Horsepen Branch is a typical piedmont headwater stream with a series of pools and riffles along its length.
Below River Road, Horsepen Branch becomes less steep and fast as it enters the Potomac River floodplain in the McKee-Besher Wildlife Management Area. Many areas of the stream are part of the Watershed Management Area (WMA) to provide a variety of wetland habitat wildlife. These wetland areas have been designated as wetlands of state concern because of their botanical diversity and value to wildlife.
The lower reaches are subject to Potomac River floodwaters that back water up the Horsepen Branch and cause bank erosion and sediment deposition into the stream. The temperature regime of the streams in the lower reaches of the Horsepen Branch might also be affected by the Potomac River.