Drainage Pattern From its source, a river flows downhill because of the earth's gravity. Small trickles of water join up to form a stream and its volume increases as more streams or tributaries join in. Eventually the stream becomes big enough to be a river. A drainage basin is all the land that supplies a river and its tributaries with water. The sequential arrangement pattern of the tributaries of a river is called its drainage pattern. Numerous analyses of the spatial arrangements of streams in drainage basins have shown that stream networks conform to a small number of stream patterns. Specific stream patterns develop in response to the initial topography of an area and the distribution of rock types of varying erosional resistance. The shape of the pattern depends on rocks, soil, climate and the changes made to the river. Drainage patterns are very often a good indication of the underlying rock types, structural features, nature of terrain and topography.
Dendritic patterns are the random orientation of channels that spread like trees with the characteristics of moderate slopes and rocks of fairly uniform resistance to stream erosion. Dendritic channels thus indicate flat-body of sediments or homogeneous crystalline rocks lacking the structural control on rocks of uniform resistance. For instance, dendritic channels are common in the great Bengal Plain, Appalachian Plateau etc.
Parallel drainage patterns occur in the same type of rocks on a steeper slope. A different type of pattern occurs when the rock units are arranged in parallel bands of alternating weak and strong lithologies. Streams preferentially exploit rocks with a low resistance to erosion, so that, in a trellis pattern primary tributaries join main streams at a right angle and the secondary tributaries parallel to the main streams. Larger streams occupy zones of non-resistant rocks, while small, short stream segments drain resistant and topographically higher beds. Tilted or folded, alternately hard and soft sedimentary rock sequences commonly display trellis stream patterns. They can also occur in regions with parallel elongate landforms such as drumlins or beach ridge. Such channels are common in the folded Appalachian and Eastern Folded Belt and the southwestern part of the barind tract of the Bengal plain. Generally alternation of sandstone and shale is common in the hilly geologic structures of the country that support the development of trellis drainage pattern.
Rectangular drainage patterns form a perpendicular net with the two directions equally developed where faults or joints are the main controlling factors. These channels are characteristic of rocks of uniform resistance cut by perpendicular joint sets. Streams preferentially occupy joint traces because of the lowered resistance caused by fracturing and weathering along joints. These channels are very common in a sedimentary hilly terrain. In Bangladesh the rectangular drainage pattern is mostly seen in the eastern hilly region where sedimentary rocks are very much controlled by structural features like faults and joints. [Kazi Matinuddin Ahmed]
See map in river.