The Lower Potomac Direct Pre-Assessment Report (43 pp, 2549 kb)
The Little Falls subwatershed is one of the County's most urban, and contains some of the oldest developed areas in the County. It drains a portion of northwest Washington, DC, the Bethesda Central Business District, and Friendship Heights. The headwaters of Little Falls Branch flow south and west approximately five miles, and are joined by Willet Branch and several other smaller tributaries, before flowing into the Potomac River. Many areas were developed before environmental regulations for stream buffers and stormwater management were put in effect. As a result, the original drainage pattern of the watershed has been extensively altered. Many of the headwater streams have been piped or channelized, and those that remain receive flow directly from high impervious areas. The downstream channels often receive flows at accelerated velocities and at elevated temperatures from traveling across heated impervious surfaces.
Historical sites such as Battery Bailey (dating back to the Civil War), the C&O Canal, an old trolley line, and the Washington Aqueduct have also helped influence the landscape in this watershed. The Little Falls Branch has also been repeatedly impacted by point source pollution events, including oil spills and leaks, chlorine discharges from drinking water treatment, and chronic sewer line problems. A 1976 study by Dieterman found no life in Little Falls; however, more recent studies have shown an improvement in macroinvertebrate and fish communities below Macarthur Boulevard. As water pollution impacts are being better managed, and stream restoration actions are completed, indications are that the stream is responding positively, especially in the lower reaches.
Rock Run is located in the southern part of the County, with its headwaters originating in a residential portion of Potomac near Falls Road and Democracy Boulevard. Rock Run has a drainage area of approximately 11 square miles. Historically, this area was impacted by gold mining activities, evidence of which may still be seen today in the form of large excavations, spoil piles, and U-shaped trenches. The subwatershed contains a mix of older residential and newer planned communities. The forested nature of the stream valleys and more effective stormwater management strategies associated with newer residential development aid in protecting the stream channel.
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