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The Great Seneca Creek Subwatershed


Image of the Great Seneca Watershed CreekGreat Seneca Creek is the largest subwatershed located entirely within Montgomery County. 

Almost every species of fish found in Montgomery County can be found in this subwatershed. Smallmouth bass have been found in the lower sections. Redbreast sunfish and central stonerollers are found throughout the middle section, and portions of the upper reaches support a cold-water fish community. 

The Great Seneca Creek joins with two other large tributary systems, Little Seneca Creek and Dry Seneca Creek, to form the Seneca Creek watershed. 


► Great Seneca Creek Subwatershed Study 1999 (PDF, 2.1MB)

► Great Seneca Subwatershed Implementation Plan (Great Seneca, Including Clopper Lake) (PDF, 2.45MB)


A Journey Through Great Seneca  

The Great Seneca headwaters begin near Hawkins Creamery Road southeast of Damascus and flow through low-density residential and agriculture areas. Magruder Branch, a large tributary that begins in south Damascus, flows through County parkland and joins Great Seneca Creek down river of Woodfield Road. The Damascus Wastewater Treatment Plant (WWTP) is in the Magruder Branch subwatershed.

Great Seneca Creek continues southwest of Laytonsville, rapidly increasing in size as other tributaries join it. Wildcat Branch, a naturally reproducing brown trout stream, and Goshen Branch join Great Seneca Creek above Brink Road. Great Seneca Creek then flows through the Montgomery Village area, where the surrounding landscape becomes much more urban and land use densities increase considerably. Many of these areas were built before modern stormwater runoff controls were required by the state, and consequently the quality of the stream channel has declined.

Below Route 355, Great Seneca Creek picks up additional drainage from high-density areas in Gaithersburg and Germantown. It then transitions back to low-density residential with areas of agricultural land from approximately Riffle Ford Road in south Germantown down to the Potomac River. Along Clopper Mill Road one can see the ruins of a mill, and old photographs at the Seneca Creek State Park record a visual history of the stream in this vicinity.

image of Macgruder Branch Trekking Trail

The quality of the stream channel has significantly degraded, with areas of active stream bank erosion and long reaches of deep runs with fewer riffle areas than observed 20 years ago.

Above Riffle Ford Road is the Seneca Wastewater Treatment Plant. Major tributaries in this portion of the Great Seneca subwatershed include Whetstone Run, Gunners Branch, and Long Draught Branch. These three tributaries originate in high- density residential areas and each has a lake: Lake Whetstone, Gunners Lake, and Clopper Lake.

Below Riffle Ford Road, some tributary areas of the Great Seneca subwatershed are changing from agriculture to low- and medium-density residential.

As the Great Seneca Creek flows westward toward Dawsonville, it is joined by Little Seneca Creek. From here, the name changes to Seneca Creek. Flowing south toward the Potomac River, Seneca Creek is joined by Dry Seneca Creek before flowing into the Potomac River above the Seneca Breaks. Many people enjoy fishing, sailing, and paddling within the mouth of the creek and out on the Potomac River.


How Can I Help Protect the Great Seneca Creek Subwatershed?

Individuals interested in helping to protect the health of the Great Seneca Creek can contact the Seneca Creek Watershed Partners. This organization of local citizens prides itself in keeping the streams in the Seneca Creek watershed healthy and alive.

    Map of Montgomery County with the Great Seneca Creek subwatershed highlighted.