DEFINITION OF DRAINAGE BASIN

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. Drainage basins 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 firstorder

streams; second-order streams form where two firstorder

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. Trellis 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 is one that

has maintained its course across topography that is being

uplifted by tectonic forces; these cross high ridges. Superposed

streams are those whose 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 GEOMORPHOLOGY.

drainage system See DRAINAGE BASIN.

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