Watersheds & Stream Order


The Environmental Protection Agency (EPA) defines a watershed as the geographic region within which water drains into a particular river, stream, or body of water. The boundary of a watershed can be identified by looking at ridges that surround the rivers. The higher elevations of mountain ridges and peaks define the boundary between watersheds. 

Stream Order

The watershed contains water in different forms - a tributary, a main channel, and a delta. A tributary is a small stream or creek. The watershed is made up of networks of tributaries, each of which flows into a larger stream. 

They are identified by stream order, determined by the order of other tributaries that have contributed to their flow. The start or headwaters of a stream, with no other streams flowing into it, is called the first-order stream. Two first-order streams flow together to form a second-order stream. Second-order streams flow into a third-order stream. This labeling continues until the streams deposit their water into a larger body of water, such as an ocean or a bay.

Identifying stream order can be useful when studying water quality.  What happens to tributaries affects higher-order streams.  Higher-order streams can contain pollutants that originate in each of its contributing tributaries.

(This information was taken from the BioSITE Curriculum website.)