What Is a Watershed?
The area of land that drains to a common body of water, whether a stream, river, or lake, is called a watershed or drainage basin. Small streams, surrounded by small watersheds, flow into larger rivers with larger watersheds. The small stream’s watershed is part of the larger river’s watershed and is called a sub-watershed. The boundaries of a watershed are the higher ridges and hills around a stream, river, or lake that separate it from the adjoining watershed.This boundary between two watersheds is called the drainage divide. From a drainage divide, water runs down gradient to a particular stream, river, or lake.

Why Are Watersheds Important?
To protect water quality, it's important to determine the boundaries of a particular watershed and define the land uses and land cover types in the watershed. The size of the watershed, the land cover types (grass, forest, pavement, etc.), and the land uses (farming, logging, residential, urban, etc.) affect the quantity and quality of water in the river, stream, lake, or groundwater. Because a single stream or river can flow through several towns, cities, counties, and even states, it is important to have all of these entities working together to protect water quality through a process called watershed planning.