Four Mile Run Program History

The Four Mile Run watershed is one of the most heavily urbanized drainage basins in the Northern Virginia region. Although the watershed is fewer than 20 square miles, it is home to approximately 156,000 people (according to the 1990 census) who reside in adjacent portions of four localities: the counties of Arlington and Fairfax and the cities of Alexandria and Falls Church.

Urbanization and Four Mile Run
Approximately 85 percent of the watershed has been developed and nearly 40 percent of the watershed is covered with impervious surfaces associated with this development (e.g., buildings and pavement). Wherever present, impervious surfaces prevent the natural process of groundwater infiltration from occurring, which in turn greatly increases the volume of surface water runoff that Four Mile Run and its few remaining tributaries must carry downstream.

The urbanization process has replaced most of the watershed's natural stream channels with an elaborate network of storm sewers. These sewers carry the increased volume of surface runoff downstream much more quickly than would the natural channels that the storm sewers replaced, which causes "flashier" and larger stream flows.

Runoff from the Four Mile Run watershed quickly makes its way into the Potomac River, and eventually drains into the Chesapeake Bay.

Flash Flooding on Four Mile Run in the 1960s and 1970s
Frequent flash flooding occurred in the Arlandria section of the watershed prior to the completion of a multi-million dollar channel modification project in 1977. This frequent flash flooding was attributed to the cumulative impacts of development from all four jurisdictions that share the watershed.

Residential and commercial areas located between the Potomac River and the Henry G. Shirley Highway (now also known as I-395) have sustained over $40 million worth of property damages (in 1968 dollars) as a result of seven major floods that ravaged the watershed throughout the 1960s and early 1970s.