Wherever you are, you're in a watershed. A watershed is all of the land area that drains water to a specific point such as a lake, bay, or section on a stream. Watershed areas are defined by topography and vary in size from small local drainage areas to large river basins. Large watersheds are composed of multiple smaller watersheds.
Depending on the water body or drainage area of interest, watersheds can be defined for large areas like the Potomac River or the Chesapeake Bay, or for small areas like those drained by very small streams.
In Loudoun County, watersheds have been defined and mapped most often at one of two sizes for use in water resource and hydrologic studies. At the larger of the two sizes, the county is divided into 17 watersheds. At the smaller of the two sizes, the county is divided into 161 watersheds (often called subwatersheds) which average about 2,100 acres each. Although these are two of the most commonly used watershed scales for Loudoun, both smaller and larger scales can be defined depending on the need.
Please note that watersheds do not necessarily follow political boundaries as is the case for Lower Goose Creek watershed which occurs in both Fauquier and Loudoun counties and for Sugarland Run watershed which occurs in both Fairfax and Loudoun counties.