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The Seneca Watershed is the largest in Montgomery County with a
drainage area of approximately 130 square miles. It includes the
subwatersheds of Great Seneca (72 square miles), as well as Dry
Seneca (19 square miles), and Little Seneca Creek (38 square miles).
(the latter two are included in the pre-assessments section)
Great Seneca
Great Seneca is primarily an urban subwatershed and flows through
various commercial, residential, and agricultural areas. The Great
Seneca headwaters begin as a series of small tributaries south and
east of Damascus near Hawkins Creamery Road, that flow through low
density residential and agricultural areas. The Great Seneca rapidly
increases in size as other tributaries join it, flowing approximately
25 miles before it enters into the Potomac River.
Distinct differences exist in the conditions found among the various
tributaries. The headwaters of Magruder Branch start in the commercial
area of Damascus and are in poor to fair condition. The nearby Wildcat
Branch is a high quality, naturally reproducing trout stream. Major
tributaries in the middle Great Seneca area all originate in high
density residential areas and each has instream impoundments (Lake
Whetstone, Gunners Lake, and Clopper Lake).
Great Seneca Summary Document and Maps 
(PDF,
4 pp, 1.1Mb)
Montgomery County Draft Great Seneca Watershed Implementation Plan (PDF, 43 pp 2.24MB)
This watershed implementation plan is unique in that it includes the small Clopper Lake subwatershed which has a TMDL for phosphorus. Even with the TMDL, there are limited identified opportunities to pursue in the Clopper Lake subwatershed, in part due to the limited area of the subwatershed within the County MS4 permit area. During the current permit cycle through 2015, the Plan prioritizes full implementation of high and low priority projects within Great Seneca Creek subwatershed. No opportunities exist for these strategies in Clopper Lake. However, full outreach was applied in Clopper Lake in the current MS4 permit cycle. In future years, the Plan proposes other potential projects such as ESD on public and private property and a small amount of riparian reforestation (in Clopper Lake). According to the model employed in the Countywide Coordinated Implementation Strategy, implementation of the Great Seneca Watershed Implementation Plan will allow Clopper Lake within the MS4 permit area to achieve the WLA for phosphorus.
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