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.