Watershed form

All watersheds share a common definition: a watershed is an "area of land that drains water, sediment, and dissolved materials to a common outlet at some point along a stream channel" (Dunne and Leopold 1978). Watershed form varies greatly, however, and is tied to many factors including climatic regime, underlying geology, fluvial geomorphology, soils, and vegetation. Subsequently, watershed form affects the form of the stream corridor as seen in longitudinal profile.

Drainage Patterns
One distinctive aspect of a watershed when observed in planform (map view) is its drainage pattern. Drainage patterns are primarily controlled by the overall topography and underlying geologic structure of the watershed.

Stream Ordering
A method of classifying, or ordering, the hierarchy of natural channels within a watershed was developed by Horton (1945). Several modifications of the original stream ordering scheme have been proposed, but the modified system of Strahler (1957) is probably the most popular today.

Strahler's stream ordering system is a well-known classification based on stream/tributary relationships. The uppermost channels in a drainage network (i.e., headwater channels with no upstream tributaries) are designated as first-order streams down to their first confluence. A second-order stream is formed below the confluence of two first-order channels. Third-order streams are created when two second-order channels join, and so on. Note in the figure that the intersection of a channel with another channel of lower order does not raise the order of the stream below the intersection (e.g., a fourth-order stream intersecting with a second-order stream is still a fourth- order stream below the intersection).

Within a given drainage basin, stream order correlates well with other basin parameters, such as drainage area or channel length. Consequently, knowing what order a stream is can provide clues concerning other characteristics such as which longitudinal zone it resides in and relative channel size and depth.