Geology Matters: Watersheds in Fairfax County

On November, 11th, Greg Bacon, an analyst at Fairfax County GIS, came to talk to the GIS 295 class about his work and the data available online at the Fairfax County Geoportal website.

The website contains information about public services, land records, land development, transportation (a big deal in Northern Virginia), safety, amenities, elections, and wildlife – basically, all the county information that your average citizen or business might need. At the very very bottom of the page, you will find watersheds.


A watershed is all of the land that drains into a particular body of water. Watersheds occur at all scales. The United States is divided into two great watersheds by the Continental Divide, a line of high peaks stretching from the Andes through the Rockies that divides land that drains into the Pacific Ocean from land that drains into the Atlantic Ocean. You can see the continental divide on the map below.continental divide

The Rocky Mountains of the U.S. formed through a series of continental plate collisions. The most recent is the Laramide Orogeny that occurred between 80 and 55 million years ago. At that time, the Rockies were about 6,000 meters above sea level.  Today, the highest peak is Mount Elbert at 4,401 meters.

While the Continental Divide determines which ocean water ultimately ends up in, there are many ways for water to get to those oceans. The U.S. Geological Survey (USGS) uses hydrologic unit codes (HUC) at six different scales (2,4,6,8,10,12) to designate the area of land that uses a particular water body as a path to the ocean. In this map, you can see HUC 2 or regional divisions. Unlike other watershed models, the USGS’s model is based entirely on water drainage – not state or local administrative boundaries.

HUC2 Fairfax County is in the HUC 2-02 watershed boundary, or mid-Atlantic region.  If I zoom in, I can see that Fairfax County is also in HUC 4-207.  Drainage in this region flows into the Potomac River. If Fairfax County was further north, water would flow into the Susquehanna. If it were further South, water would flow into the lower Chesapeake. The upper Chesapeake watershed is to the east.huc4

At the HUC 6 level, most of Fairfax flows into the Middle Potomac. However, the rivers used to  get to the mid-Potomac differ. That causes the area to be divided into the Anacostia-Occoquan and the Catoctin district (northwest).

The area continues to be subdivided until the HUD 12 level which is based on local creeks and lakes – a resolution that is very similar to the Fairfax County dataset. The Fairfax County system continues to subdivide regions to the level of individual creeks and streams, but one must click at the “more info” tab on the mapped watersheds to learn about these subdivisions.


There is one important difference between the Fairfax County watershed designations and the USGS system. The Fairfax County watershed boundaries end at the county line. The USGS boundaries do not. That is because the USGS watersheds are based on geology. The Fairfax County watersheds are based on administrative boundaries.

I looked up the Cob Run and Bull Run watersheds on the Fairfax County website. This area is located in southwestern Fairfax County between Loudon and Prince William Counties and covers 64 miles of land that drain into tributaries of the Occoquan Reservoir.  The website says that 14 square miles in Loudon County also drains into this watershed.  This matters.

We all need clean water. Yet, the conveniences of everyday life (manufacturing, transportation, agriculture) create pollution that is carried into our drinking water. In Fairfax County, all drinking water comes from the Potomac or the Occoquan.

All water that falls or accumulates in a river’s watershed goes into that river. This includes the rain that falls on the roads and mixes with the oil and gas that cars leave behind It includes chemical-laden runoff from industry and farms. It even includes the water that washed off your neighbor’s yard after he sprayed his azaleas. Although, this water is cleaned and treated, small “safe” amounts of contaminants remain.

Fairfax County, like all counties, must consider water pollution when planning for the future. The County must answer questions like “How will expanding Route 28 increase runoff into Bull Run?”,  “How much of that water will end up in our drinking supply?”, and “How will that water be treated?” What happens when Manassas doesn’t care because commuters are sick of sitting in traffic and the creek is across a county line? Or, when an administrator uses the map without reading the metadata? Wouldn’t it be better to stick with the USGS system which is based on the way the Earth actually works and then figure out the overlap?

As a geologist, I know that a watershed is all of the land that drains into a particular body of water.  I’m uncomfortable with the Fairfax County map; anyone want to offer reassurance?

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s