The main water supply system for Washington,D.C. and some of its suburbs. It was originally bult in the late 19th century (by what is now the Army Corps of Engineers) to supply tap water to a city that was rapidly growing, and had no other clean water supply.
The centerpiece of the system is a long aqueduct, composed of two tunnels leading from a masonry dam at Great Falls. It winds its way through the southern part of Montgomery County, Maryland, with MacArthur Boulevard (originally built as a service road) paralleling it most of the way, until it reaches Dalecarlia Reservoir, home to one of D.C.'s two filtration plants. Some of the water is also pulled from a point lower on the Potomac, at Little Falls, which is just upstream of Dalecarlia.
Some of the water also goes on to Georgetown Reservoir, which is used as a settling basin much like certain of the reservoirs in New York City's system are. From there, water goes through the Washington Water Tunnel to McMillan Reservoir, and D.C.'s second filtration plant.
Most of the aqueduct's product is used by D.C. itself, but it also suplies water to part of Virginia (most notably Arlington County, Virginia, but also Falls Church, VA and Vienna, VA).