Stream Order

Stream Order

Stream order refers to systems for classifying streams based on branching patterns, starting at the headwaters. Stream order is closely related to stream size -- the smallest streams are the lowest order and stream size increases with stream order. In the most commonly used scheme, the uppermost headwater streams that have no tributaries are first-order streams (Strahler 1952). When two first order streams join, they form a second-order stream. Likewise, when two second-order streams join, they form a third-order stream, and so forth.... When a lower order stream joins a higher order stream (for example, a first-order stream joins a second-order stream), the order of the receiving stream does not change. When a stream is assigned an order, the number refers to the lowermost section of the stream.

Some confusion, however, arises because scientists use different criteria for determining where first-order streams begin in the headwaters. Some headwater streams are ephemeral, with short-lived or transitory flow after precipitation or snowmelt and others are intermittent-- they flow at certain times of the year, cease flowing in dry years, or are reduced to a series of separated pools. Nevertheless, these streams are important for some invertebrates and for stream ecosystem processes. Strahler (1952) evidently intended to begin with ephemeral and intermittent streams. Hydrologists include all these streams when assigning stream order and tend to use crenulations in the contours on a 1:24,000 USGS topgraphic map to identify first order channels. For some fish biologists, only streams with perennial flow are of interest, and the uppermost perennial stream is designated as first order. For convenience, many people define first-order streams as the uppermost streams shown by blue lines on a 1:24,000 USGS topographic map, which generally includes most perennial streams, some intermittent streams, but rarely ephemeral streams.