Page updated on May 23, 2018 at 11:32 AM

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:  like 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.  Its 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.  Does that make sense?!  So what is our watershed address?

Know Your Watershed

WatershedMapSmallPicMost of us may know that we're in Potomac River Watershed, inside the Chesapeake Bay Watershed.  But do you know your local watershed address?  In the City of Alexandria, we recognize eight local watersheds that drain the land and subsurface.  Here's a map of the City's watersheds.  See if you can locate your house, school or office and find your "watershed address(es)".