Water soaks into soil and flows underground to the lowest point the rocks will allow. This subterranean water is called ground water. Some water is not absorbed by the soil, and we call this surface water. Surface water can take on many forms including rivers, streams, wetlands and lakes. In many parts of the state, surface water and ground water interact.
Watersheds occur in all different shapes and sizes.
Hydrologic Units generally follow the watershed boundaries around our major rivers but, sometimes, they don't exactly match. Watersheds are defined by ridges. If you could stand in a river and watch the rain strike the exact top of the ridge, you would notice that the raindrop either flowed downhill into your watershed or over the ridge into an adjacent one. In Tennessee, the ridges defining watersheds are much more obvious in East and Middle Tennessee than in West Tennessee, which is predominantly flat. In West Tennessee, some watersheds are prone to flooding because the land is so flat that water covers adjacent fields when rivers top their banks.