Most of the rivers originate in the mountains. Initially the water moves down the slope in the form of rills or streams. In due course of time, a number of small streams join the main stream.
The main stream, with all its tributaries, develops a drainage system. The total area occupied by a drainage system is known as its drainage basin. The higher ground, which separates the two drainage basins, is called the water-shed or the water-divide.
Originally, when the streams flow in the direction of the slope of the land, or as a consequence of the slope such streams are called the consequent streams. As soon as such a stream is joined by a tributary, that tributary is called the subsequent stream.
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The drainage pattern means the geometrical forms of the drainage system and the special arrangement of the streams in a particular region. The location, number and the direction of flow of various streams in a region depends on the nature of the relief, shape of the land, climatic conditions, etc.
The drainage patterns actually reflect the relief of the area, the nature of the rocks, the slope of the land and probably the amount of rainfall in the area. Since there is much variation in the environmental conditions of different special regions, there are variations in the drainage patterns. But there are some common characteristics, which enable us to distinguish the different drainage patterns. The most common drainage patterns are given below:
1. The Dendritic Drainage Pattern:
The word ‘dendron’ in Greek means a tree. In this tree-like pattern, the main river is like the trunk and the tributary streams join it like the branches of a tree. The tributaries do not meet the main stream at right angles.
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The presence of the same type of rocks in extensive areas provides the ideal conditions for its development and growth. This pattern is generally noticed in flat regions. It is the most common and widespread drainage pattern.
2. The Trellis Drainage Pattern:
The tributaries join the main stream at right angles and thus form a trellis. This pattern is mostly developed in areas of simple folded structures, with parallel anticlinal ridges alternated by synclinal valleys. It has long and almost straight river valleys, with their tributaries, coming from the anticlinal ridges, joining almost at right angles.
3. The Rectangular Drainage Pattern:
This pattern has resemblance with the trellis drainage pattern, as the tributary streams in both cases, join the main stream almost at right angles. The rectangular pattern develops generally in those areas, where the rock joints (cracks, faults and fractures) form a rectangular pattern. Another difference is the spacing between the tributary streams. In the trellis pattern, the streams are closely spaced while in the rectangular pattern, the streams are widely spaced.
4. The Radial Drainage Pattern:
This drainage pattern develops when a number of streams radiate from a central high region in all directions like the spokes of a wheel. These are basically the consequent streams as they follow the slope of the land. The dome structures, volcanic cones, residual hills, small tablelands and isolated uplands favour the development and growth of the radial drainage pattern.
5. The Centripetal Drainage Pattern:
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This drainage pattern is just opposite to the radial drainage pattern, as the streams generally converge at a point, which is either a depression or a basin. The streams emerge from the surrounding uplands.
6. The Parallel Drainage Pattern:
This drainage pattern comprises a large number of rivers, which flow parallel to each other and follow the slope of the region. It is ideally developed in the uniformly sloping beds.
7. The Annular Drainage Pattern:
It is also known as the circular drainage pattern. This drainage pattern is formed when the tributaries of the main consequent stream are developed in the form of a circle. The main stream emerges from the top of a dome, whereas the tributary streams develop in the fissures formed due to the erosion of the soft beds.
8. The Inland Drainage Pattern:
In some areas, due to physical and climatic conditions, the streams are unable to reach the sea, or the ocean, or a lake. This also includes the disappearing drainage pattern, where the streams merge in the desert sands and fail to reach the sea coast.
On the topo sheets, the streams or rivers which carry water throughout the year (the perennial rivers) are shown in blue colour. The smaller streams which become dry during a part of the year (seasonal rivers) are shown in black colour. The irregular streams, which disappear in the deserts, are shown as black dotted lines.