A watershed is an area of land where all water drains into a common waterway, be it a stream, river, lake, wetland, estuary, or even the ocean. Since all water runs downhill due to the force of gravity, watershed boundaries are typically comprised of ridge tops or high elevation areas.
A watershed can be very large and can cover several states. For example, the Chesapeake Bay watershed encompasses over 64,000 square miles, and consists of parts of New York, Pennsylvania, Maryland, West Virginia, Delaware and Virginia. Watersheds can also be very small, encompassing a few small streams or wetland areas. Charlottesville lies in the Rivanna River watershed, which is a medium sized watershed, encompassing 766 square miles. The Rivanna River watershed is nested within the James River watershed, which lies within the even larger Chesapeake Bay watershed.