Stormwater and Watersheds
A watershed is an area of land that drains to a common body of water such as a stream, river or lake. Excluding water taken in by plants, all the rain that falls on Arlington eventually drains into the Potomac River. The 26 square miles of the County are a small part of the 64,000-square-mile Chesapeake Bay watershed. View Arlington’s watershed map.
Stormwater is rainfall that doesn’t soak into the ground. Instead it flows over land into the County’s extensive storm drain network of pipes where it’s released — untreated — into our streams.
As the percentage of impervious surface increases, less water is able to soak into the ground, and more runs off as stormwater.
Stormwater Background
Approximately 44 percent of the County’s land is impervious. Impervious surfaces, like roads, sidewalks and buildings, don’t allow water to soak into the ground. View the impervious cover amounts by watershed(PDF, 157KB) (2021 data).
View a recent County Board work session about impervious cover trends and tree canopy in low density residential areas in Arlington.
In the early decades of Arlington’s expansion (1930-60s), there was very limited regulation for development. The natural stream network served as the stormwater management system. But as swift development caused stream erosion and gullies, streams and floodplains were filled in or paved over. 66% of streams were encased in stormwater pipes. The stream network could no longer convey flood waters safely during storms.
The County’s original storm sewer pipe network was designed for approximately a 10 year storm. Current building standards require that overland relief be required in addition to the pipe network, to provide a safe flow path for water to the nearest stream when a storm exceeds the capacity of the pipe network. However, in in some places in Arlington, overland relief was not required at the time of development, and homes were built near the location of the former streams (now encased in storm sewer pipes).
In addition, most stormwater in Arlington is not filtered or treated before it flows into local streams. However, we’re working hard to improve our stormwater infrastructure, and to install sustainable stormwater management techniques to improve water quality.
Advent of Stormwater Regulation
Arlington adopted its first Stormwater Master Plan in 1957. Regulations requiring stormwater management were initially adopted in the 1970s at both the State and Federal level. However, by the time the regulations came into effect, much of Arlington was already developed.
Early stormwater regulations required stormwater detention for construction projects, such as Ballston Pond, which was built to collect runoff from I-66. More recent stormwater regulations require pollutant removal as well as detention of stormwater.
Arlington’s stormwater programs are part of the Office of Sustainabilty and Environmental Management in DES, including water quality regulations, planning engineering, construction, stormwater infrastructure, and incentive and outreach programs.