We have defined drainage pattern and discussed both the dendritic drainage pattern and the trellis drainage pattern. The DENDRITIC DRAINAGE PATTERN was a tree-like trace of the streams on a map that resulted from uniform surface control of the development of stream channels. The bedrock is of equal resistance to erosion. Given an absence of other structural controls, a stream will tend to develop the dendritic drainage pattern. We have also mentioned the trellis drainage pattern. The TRELLIS DRAINAGE PATTERN is associated with eroded folds where resistant and non-resistant rocks are interbedded so that the resistant rocks such as some sandstones form ridges and non-resistant rocks like shale and limestone form valleys. The tributary streams flow down off of the resistant ridges and join higher order streams flowing along the valleys. Six other drainage patterns are often distinguished. An ANNULAR DRAINAGE PATTERN is similar in that it is a bent trellis. When the eroded folds form domes or basins, the ridges and valleys are circular, and so the trellis shape is bent around parallel to the plunging folds. A RECTANGULAR or ANGULAR DRAINAGE PATTERN is the result of structural control joints or faults in the bedrock. These breaks in the rock are weak areas, so the streams tend to flow along these areas. Fault and joints are linear features, often in parallel trending sets. Thus, tributaries join at right angles with parallel trending branches. RADIAL DRAINAGE PATTERNS originate from streams flowing OUT from a common center, such as off of a volcanic cone. The opposite pattern is a CENTRIPETAL DRAINAGE PATTERN, where the streams flow IN to an interior lowland. This is common in areas of the Basin and Range province of the western United States. The southeast coast of the U.S. exhibits the PARALLEL DRAINAGE PATTERN. This trace is straight, parallel stream channels that develop on a pronounced regional slope. Shoreline changes have caused these streams to keep extending themselves further toward the sea inch by inch through the centuries, and this results in the parallel channels through the coastal province. If the surface drainage is sufficiently disrupted, such as by extensive inundation by volcanic material or by glaciation, the integration of stream channels will be destroyed and will take time to become re-established. This pattern is referred to as a DERANGED DRAINAGE PATTERN.
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