Stream Order
A simple method has been developed to categorize streams in a watershed (see Figure 4-4). Streams that have no tributaries flowing into them are called first-order streams or headwater streams. These streams typically begin as a spring, or as an outlet from a lake, pond, or wetland.
A first-order stream joined by another first-order stream becomes a second-order stream (labeled “A” in Figure 4-4). When two second-order streams join, the result is a third-order stream (labeled B). Two third-order streams result in a fourth-order stream (labeled C), and so on. This process continues until all the streams in a watershed merge into the largest river, which ultimately drains into a lake or ocean. The Mississippi River, near its mouth, is a twelfth-order stream. Keep in mind that two like streams must join to form the next highest stream order. For example, if a second-order stream flows into a third-order stream. The joined flow is still a third-order stream (labeled D).