The lower portion of Four Mile Run, from I-395 at the upstream end to the mouth at National Airport, marks a rough boundary between Arlington County and the City of Alexandria. Along this stretch of Four Mile Run are neighborhoods, commercial districts, and some industrial facilities, including the Arlington County Water Pollution Control Plant.
Because of the highly urbanized nature of the Four Mile Run watershed, the neighborhoods and businesses adjacent to this portion of the run were subjected to repeated flooding, beginning in the 1940s.
In response to this flooding, the municipalities forged a partnership with the US Army Corps of Engineers (USACE) to build a flood-control channel in the lower portion of Four Mile Run. Construction of that channel took place during the 1970s and early 1980s.
Since its completion over twenty years ago, the channel has safely conveyed the high storm flows through the two jurisdictions.