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.)