A watershed is a land area that funnels rainfall or snowmelt to a watercourse or larger body of water.
A watershed can be minute in scale, consisting of just a small lake or a single county. However, some watersheds extend for thousands of square miles and can include streams, rivers, lakes, reservoirs, and underlying groundwater.
When rain falls, it can soak into the ground or may run off the surface. Water that seeps into the soil will eventually make its way into the nearest stream. Some water infiltrates much deeper, into underground reservoirs called aquifers. In areas where the soil contains hard clay, very little water may infiltrate the ground. Instead, it quickly flows to lower ground.
Even parking lots, roads, and buildings are part of a watershed. During periods of heavy rain water runs off these impervious surfaces and into storm drains. These storm drains funnel that water into streams and rivers where it can overwhelm them, causing them to overflow and possibly flood.