Page updated on Jul 7, 2021 at 4:52 PM

What is a Watershed?

A watershed is an area of land where all the water that drains off of it or under it goes to the same water body, such as a creek, river, estuary, wetland, reservoir, lake, or ocean. A watershed boundary is the extent to which the surface runoff and subsurface flows are collected by the topography and travel to the water body.  The topographical boundary (or drainage divide) can be a crest, hill or a railroad track that separates one watershed from another. We typically use the watershed to describe an entire drainage basin and often use the name of the most prominent water feature as its name. 

We all live in a watershed. It's often a matter of perspective and scale when we talk about watershed boundaries. You can think about watersheds like bowls sitting next to each other, as well as other bowls being nested (or sitting) inside of the bowls. Because of this characteristic, we may have a different local watershed address than our neighbor down the street who lives in Alexandria. However, the whole City is in the Potomac River Watershed. Confusing? Let's see if we can clear it up a bit. 

Think About This!
What if you were to hold your hands together like a bowl while standing in the rain or under a sprinkler. All the water that falls in your "bowl" ends up at the bottom of your hands.  That's your watershed!  Now have your friend stand beside you and make a "bowl". Some of the water will fall inside their hands. That's their watershed. Now stand next to each other and place your "bowls" next to each other and you can see how adjacent watersheds work. Oh, and while both of you have your own "watershed address", you're both inside the larger "watershed address" that is the house or the lawn.

Know Your Watershed


The City of Alexandria is about 15 square miles and boarded on the east by the Potomac River. The Potomac River is one of the five major rivers draining to the Chesapeake Bay. The other tributaries are the Susquehanna, Rappahannock, James, and York rivers. The Potomac River is 405 miles long and drains 14,700 square miles of land which is about 22% of the entire 64,000 square mile Chesapeake Bay watershed. To learn more about the Chesapeake Bay, click here

Tidal Influence

The “tidal Potomac River” begins at Chain Bridge and ends where the Potomac enters the Chesapeake Bay. There are three characteristically distinct segments of the tidal Potomac River with the first segment being from Chain Bridge to Quantico. This segment of the Potomac, which travels past the City of Alexandria, is characterized with “freshwater flows and riverine chemistry”. As the Potomac travels to the Bay, the river starts to mix the freshwater with the saltier water of the Chesapeake which is the largest estuary in the U.S. An estuary is a waterbody that combines the freshwater from rivers with the salt water from the ocean.

The federal National Oceanic and Atmospheric Administration (NOAA) maintains a station on the Potomac River (Station ID: 8634214) that predicts the tides of the Potomac River. United States Geological Survey (USGS), in partnership with Alexandria, maintains several USGS monitoring sites in Alexandria that track the water surface elevation over time. USGS National Water Information System Real-Time Stream Flow Data stations: Cameron Run (USGS Station); Four Mile Run at Route 1 (USGS Station 01652545); and Cameron Run a little west of Route 1 and I-495 (USGS Station 0165258890). 

Local Watersheds

In the City of Alexandria, we recognize eight local watersheds that drain the land and subsurface. These are:  Holmes Run, Strawberry Run, Taylor Run, Four Mile Run, Cameron Run, Backlick Run, Hoof's Run, and the Potomac River. Here's a map of the City's local watersheds. You can find the exact watershed where your home, business, school, or office is located by using the Watershed Tool, which allows you to type in any address. You can also use the City's online GIS Sewer Viewer to find the storm sewer line (green) to trace what water body your stormwater drains to.

Surface Water Quality

Our drinking water is managed by Virginia American Water and our waste water is managed by the City's Sanitary Sewer Division and treated by AlexRenew. Our surface water is impacted by stormwater runoff and other factors. These waters are tested and categorized by the state for water quality standards established by the federal Clean Water Act. When the water body does not meet the standard, it is considered to be "impaired". You can learn more about the Virginia Water Quality Assessment here and the City's plans to restore the health to our local waters here.

Enjoying our streams safely

While the City has programs to protect local watersheds, stream water can contain micro-organisms that can make people sick, whether the stream is located in an urban area or in the middle of a forest. Learn some tips on enjoying our streams safely here.

What you can do to protect water quality

There is a lot the the City does to help protect water quality such as education and outreach and stormwater management. Click here to learn what you can do to help protect water quality.