Stream Erosion and Deposition

Streams are predominant shapers of the terrestrial landscape. Main streams and their innumerable tributaries are responsible for much of the landscape we see. Even in arid areas, stream water can do significant work when available because of the lack of protective vegetative cover fostering increased surface erosion and runoff. In the absence of significant tectonic disturbance, the overall effect over very long amounts of time is to make the higher places lower by erosion and lower places higher thus “smoothing” the landscape. Within the immense time scale related to smoothing it is common for streams to produce hills, valleys, and canyons by lengthening, deepening, and widening their valleys.

The landscape has been given a simple classification vis-a`-vis stream erosion. The surface area drained by a stream is a drainage basin (or a watershed) and its boundaries are drainage divides. Drainage basins exist on vastly different scales and, because streams have tributaries, main streams are associated with the multitude of tributary drainage basins that are nested at various scales one within another. It is through the organization of drainage basins that the flows of mass and energy associated with streamerosion and deposition can be understood. Stream-produced landscapes are segmented into interfluves, valley sides, and valley bottoms. High ground dividing adjacent river valleys is known as an interfluve. At the highest elevation along an interfluve is a drainage divide. Interfluves can be many kilometers across or nonexistent in the case of drainage basins edged by steep mountains. The valley side is the increased slope down to the valley bottom. Valley sides can have slopes of only a few degrees from horizontal or be bluffs or canyon sides. The valley bottom is the surface along which a stream is eroding or depositing and can be negligible such as on a canyon bottom or many kilometers wide.