Further Reading

Kious, Jacquelyne, and Robert I. Tilling. "U.S. Geological Survey. This Dynamic Earth: The Story of Plate Tectonics." Available online. URL: http://pubs.usgs.gov/ gip/dynamic/dynamic.html. Updated March 27, 2007. Skinner, Brian, and B. J. Porter. The Dynamic Earth: An Introduction to Physical Geology. 5th ed. New York: John Wiley & Sons, 2004.

drainage basin (drainage system) The total area that contributes water to a stream is called a drainage basin, and the line that divides different drainage basins is known as a divide (such as the continental divide) or interfluve. Drainage basins are the primary landscape units, or systems, concerned with the collection and movement of water and sediment into streams and river channels. They consist of a number of interrelated systems that work together to control the distribution and flow of water within the basin. Hillslope processes, bedrock and surficial geology, vegetation, climate, and many other systems all interact in complex ways that determine where streams will form and how much water and sediment they will transport. A drainage basin's hydrologic dynamics can be analyzed by considering these systems along with how much water enters the basin through precipitation and how much leaves the basin in the discharge of the main trunk channel. Streams are arranged in an orderly fashion in drainage basins, with progressively smaller channels branching away from the main trunk channel. Stream channels are ordered and numbered according to this systematic branching. The smallest segments lack tributaries and are known as first-order streams; second-order streams form where two first-order streams converge, third-order streams form where two second-order streams converge, and so on.

Streams within drainage basins develop characteristic branching patterns that reflect, to some degree, the underlying bedrock geology, structure, and rock types. Dendritic or randomly branching patterns form on horizontal strata or on rocks with uniform erosional resistance. Parallel drainage patterns develop on steeply dipping strata, or on areas with systems of parallel faults or other landforms. Trel lis drainage patterns consist of parallel main-stream channels intersected at nearly right angles by tributaries, in turn fed by tributaries parallel to the main channels. Trellis drainage patterns reflect significant structural control and typically form where eroded edges of alternating soft and hard layers are tilted, as in folded mountains or uplifted coastal strata. Rectangular drainage patterns form a regular rectangular grid on the surface and typically form in areas where the bedrock is strongly faulted or jointed. Radial and annular patterns develop on domes, including volcanoes and other roughly circular uplifts. Other, more complex patterns are possible in more complex situations.

Several categories of streams in drainage basins reflect different geologic histories—a consequent stream is one whose course is determined by the direction of the slope of the land. A subsequent stream is one whose course has become adjusted so that it occupies a belt of weak rock or another geologic structure. An antecedent stream has maintained its course across topography that is being uplifted by tectonic forces; these cross high ridges. Superposed streams' courses were laid down in overlying strata onto unlike strata below. Stream capture occurs when headland erosion diverts one stream and its drainage into another drainage basin.

See also estuary; fluvial; geomorphology; river system.

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