Drainage basins and drainage patterns

A drainage basin is the area that encompasses all the land from which water flows into a particular stream or river. Stream is a synonym of river, and although typically something called a stream is smaller than a river, here, any flowing body of water in a clearly defined channel will be called a stream. The size of a drainage basin can vary from being as small as a few square miles or kilometers to as large as part of a continent. An example of a divide is the continental divide of North America , which separates streams that ultimately empty into one ocean (the Pacific Ocean) from those that ultimately empty into another (the Gulf of Mexico ). The smallest streams in any particular area are called first order streams, and the land from which water flows into a particular first order stream is called a first order drainage basin. First order streams flow into second order streams, and each second order stream has its own second order drainage basin. There is no limit to how high an order a stream may be.

The drainage pattern that streams in a drainage basin trace out, visible in aerial photographs or even from the window of an airliner, can provide a lot of information about the type of terrain that the streams flow over. The dendritic drainage pattern of streams resembles the veins of a leaf, or the structure of a tree. It typically develops in areas with homogenous or flat-lying rocks that provide no preferred direction to the development of stream channels. Streams that flow over the flat-lying rock units of the American Midwest often display this type of drainage pattern. An annular drainage pattern forms when layers of rock are uplifted into a dome or down-warped into a basin, and the stream channels preferentially follow the weakest concentric beds of rock. A radial drainage pattern develops where there is a central highpoint, such as an isolated volcanic peak. The streams all flow away from the highest point. Fractures in massive rock such as granite can produce a drainage pattern in which the streams have many right-angle turns, and this is called rectangular drainage. When layered rock units are folded or tilted up, lower-order streams that flow into larger streams tend to be straight and follow weaker beds of rock. This trellis drainage pattern is common in the Appalachian Mountains of the United States. Centripetal drainage is found where streams flow into the center of a depression such as a basin or crater. Deranged drainage forms on terrain that is freshly exposed, and where the streams have not had a chance to develop in response to underlying geologic structure or bedrock . Finally, parallel drainage tends to develop in areas of massive rock with a uniform slope, where all the streams tend to flow in the same direction.

See also Avalanche; Delta; Drainage calculations and engineering; Hydrogeology; Runoff

drainage basin morphometry The measurement of the characteristics of the surface form of a drainage basin (catchment), and of the arrangement and organization of the associated river network. Properties such as area, shape, gradient, and relief are important elements of form (see drainage basin shape index and drainage basin relief ratio), while the stream network is investigated through a study of its components and of the ways in which they are related. See drainage network analysis.

drainage basin shape index A measure of the shape of a drainage basin (catchment), normally expressed as the ratio between two dimensions of the basin being considered. One such measure is the circularity index (or ratio), C, expressed as C = A_b/A_c, where A_b is the area of the basin and A_c is the area of a circle with the same length of perimeter as the basin. Another index is the form factor, F, expressed as F = A/L, where A is the area of the basin and L is its length. Such indices may help in forecasting the flood potential of a basin.

drainage basin shape index A measure of the shape of a drainage basin, normally expressed as the ratio between two dimensions of the basin being considered. One such measure is the circularity index (or ratio), C, expressed as C = A_b/A_c, where A_b is the area of the basin and A_c is the area of a circle with the same length of perimeter as the basin. Another index is the form factor, F, expressed as F = A/L², where A is the area of the basin and L is its length. Such indices may help in forecasting the flood potential of a basin.

drainage basin morphometry The measurement of the surface form of a drainage basin, and of the arrangement and organization of the associated river network. Properties such as area, shape, gradient, and relief are important elements of form (see DRAINAGE BASIN SHAPE INDEX; and DRAINAGE BASIN RELIEF RATIO), while the stream network is investigated through a study of its components and of the ways in which they are related. See DRAINAGE NETWORK ANALYSIS.