Introduction
This page provides an overview of watersheds and urban stormwater, explains the problems caused by stormwater here in Arlington and the County's approach to addressing these problems, and suggests ways County residents can help improve the conditions in County streams.
What is a watershed? 
A watershed is all the land that drains into a water body, such as a stream, river, lake, or estuary. All of the rain that falls on Arlington County eventually drains to the Potomac River, except the water that is taken up by plants. On a somewhat smaller scale, drainage in Arlington County flows either directly to the Potomac River or into two tributaries of the Potomac: Four Mile Run and Pimmit Run. Within these two areas, subwatersheds like Gulf Branch, Donaldson Run, and Windy Run flow into the Potomac, and subwatersheds such as Lubber Run and Upper Long Branch flow into Four Mile Run. View a map of Arlington's watersheds.
On a much larger scale, Arlington's 26 square miles are a small part of the 64,000 square mile Chesapeake Bay watershed.
What is watershed management? By focusing on activities in the entire area that drains to a waterbody, watershed management programs take a holistic approach to protecting streams, rivers, lakes, and estuaries. As such, watershed management programs often require inter-jurisdictional cooperation, as well as cooperation among government agencies and among governments, businesses, developers, and citizens.
Watershed management programs include land use planning, regulation of development, control of water pollution, stream buffer protection and stream restoration, and outreach and education. Find out more about Arlington's Watershed Management Programs. These programs consider all sources of pollution in a watershed, including spills and leaks, factories, and stormwater runoff from urban and agricultural areas. In an urban area like Arlington, with very few industrial facilities, stormwater is the main source of pollution to local streams. The major point source discharger in Arlington is the Water Pollution Control Plant.
What is stormwater?
Stormwater is rainfall that flows over the land surface to the nearest water body, instead of soaking into the ground. In urban areas such as Arlington, much of the land is covered by hard surfaces such as streets, buildings, and parking lots. These hard surfaces do not absorb rainwater. In contrast, undeveloped areas such as forests and meadows soak up most of the rain that falls on the ground. Therefore, there is much more stormwater runoff in areas with large amounts of impervious surface.
Stormwater is sometimes called nonpoint source (NPS) pollution, because it comes from many different sources, unlike pollution from factories and sewage treatment plants that originates from a pipe. Other types of NPS pollution include stormwater runoff from agricultural lands and logging operations. NPS pollution is the Nation's largest source of water quality problems (find out more about NPS pollution). Learn more about Arlington's Stormwater Management Programs.
Where does urban stormwater go?
Rain that falls on your house, apartment building, business, or office building almost immediately becomes stormwater runoff. This runoff flows down roof gutters and combines with stormwater runoff from driveways, lawns, streets, and parking lots. Most of this water then flows into a nearby storm drain, and into the storm sewer system. Arlington's storm sewer system consists of 360 miles of underground pipes that carry stormwater directly to the County's streams.
Although urban stormwater is NPS pollution, because this water is usually discharged to surface water through discrete outfall pipes from the storm sewer system, it is regulated as a point source (find out more about how urban stormwater is regulated).
Is urban stormwater a problem?
Yes, for several reasons. The first major problem is created by the sheer amount of stormwater runoff in developed areas as a result of impervious surfaces. This stormwater is concentrated in the storm sewer system and carried to streams at very high velocities. When this stormwater exits a storm sewer outfall pipe, it blasts the adjacent stream channel, widening and deepening the channel. The soil eroded from streambanks during storms smothers aquatic habitat in local streams like Lubber Run, Gulf Branch, and Four Mile Run. Over time, this sediment is carried downstream to the Potomac River and the Chesapeake Bay, combining with sediment from other urban streams in the Bay watershed. This sediment is one of the main reasons that the Chesapeake Bay is degraded. And, eroded stream channels in Arlington's stream valley parks also diminish the recreational experience for County residents.
The second major problem caused by stormwater is what's in it. Stormwater picks up a variety of pollutants that accumulate on streets, parking lots, buildings, and even lawns, such as petroleum, metals, pesticides, nutrients, and bacteria. These pollutants can cause a variety of problems for organisms, including humans, that live in or interact with streams, rivers, and other water bodies.
Another critical problem is the temperature of stormwater during the summer months. Impervious surfaces such as streets, driveways, and parking lots become very hot in the summer. During a typical summer afternoon thunderstorm, the stormwater that flows across these hot surfaces picks up a lot of heat and can cause the temperature of an urban stream like Four Mile Run to rise by as much as 10 degrees Fahrenheit during and after a storm. This temperature change is enough to kill many of the aquatic organisms that live in the stream.
Yet another problem is related to the first problem. Because so much rainfall in developed areas is quickly conveyed to the nearest stream during storms, much less water soaks into the ground to slowly supply streams with groundwater during dry periods. As a result, when it is not raining, water levels in streams tend to be lower in developed areas, further degrading aquatic habitat and overall stream health.
Nationwide, urban stormwater has degraded water quality in more than one-fifth of the rivers and streams in the U.S.
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