What is a Watershed?

Watersheds are defined by higher peaks and elevation in the landscape, and by the movement of water among landscapes.

When precipitation falls to the ground, it is the area of land that absorbs the water into the ground (groundwater) and also “sheds” or drains the water to lower elevation through rivers and streams (surface water).

Watersheds describe how units of land are connected by water flow.

Headwater watersheds are the most upstream watersheds that transform precipitation (rain, snow, sleet, anything that falls from the sky) into streamflow which eventually connect to larger bodies of water such as rivers, lakes, reservoirs, catchments, wetlands and, eventually, the ocean.

Headwater watersheds are often forested (or once were prior to agricultural expansion). As water naturally travels through the headwater watersheds to the flood plains and valleys, it erodes the surface it is traveling on, taking with it large amounts of sediment, metals, and other compounds that it comes into contact with.

A useful excerpt from the USDA handbook on stream restoration:

Watersheds are nested within one another, with larger watersheds composed of many smaller tributary watersheds, and these smaller tributaries drained by even smaller intermittent channels, ephemeral channels
and rills. Watersheds are comprised of a mosaic of soil types, geomorphic features, vegetation, and land uses. If a watershed is divided into uplands and stream corridors, the uplands comprise most of its area (in most basins). Upland features control the quantity and timing of water and materials that make their way to
the stream corridor.
The environmental conditions of the stream corridor (such as water quantity and quality,
riparian function, and fish habitat) are, therefore, linked to the entire watershed, and these linkages go both ways. For example, animals living primarily in upland habitat frequently rely on stream corridors for movement, food, cover, and water. Although stream project designers may have little or no control over how a watershed is managed, their plans and designs still should consider the past, present, and future status of watershed land use and historical watershed conditions.


Watersheds are divided and subdivided into “hydrologic units” which identify the size and scope of the watershed. The United States is divided into 21 major regions (largest geographic region, depicted by a 2-number hydrologic unit code, or HUC2) shown in the image below:


Maps retrieved from USGS website: https://nas.er.usgs.gov/hucs.aspx

These 21 units are further subdivided into smaller hydrologic unit codes, the higher the unit code number, the smaller the region: HUC4, HUC6, HUC8, and HUC12.

The Lake Helena Watershed is classified as a HUC12 watershed. Click Here: The Lake Helena Watershed to learn more.