Course of a River
The course of an ideal river may be divided into three parts according to its mode of work:
The upper course or mountain course,
The middle or the plain course and
The delta or the lower course.
- The upper course or mountain course:
Erosion is the principal form of work in mountains. The major types of landforms produce by a river in its upper course are:
- V-Shaped valley and Gorge: The valley of a river is ‘V’ shape viewed sidewise. The shape is produced by the joint action of vertical erosion or down cutting and side wash or lateral erosion. The down cutting of the river deepens the valley whereas the side wash make the valley sloping upward away from the river. As the slope of the land is steeper and the velocity higher in mountain, vertical erosion or down cutting of a river far exceeds lateral erosion. That is why the cross profile of a valley assumed typical ‘V’ shape. In some cases, with deep down cutting and deep vertical sides, a narrow valley called Gorge is produced. The divide at the intersection of two adjacent valleys (or two ‘V’s) becomes narrow and sharp.
- ‘I’ shaped valley or canyon: Under some special circumstances narrow ‘I’ shaped valley or canyons are formed in the course of a river. In deserts due to scanty rain-fall, lateral erosion is restricted but vertical erosion continues. This results in the development of an ‘I’ shaped valley or a canyon of a river.
- Interlocking spur: The sloping uplands running in between valleys of tributary streams issuing from the main water divide are called spurs. The main river at the valley bottom meanders through such interlocking spurs.
- Waterfalls: Waterfalls are characteristic features in mountain course of a river. The chief causes of their formation are as follows: In some cases a hard rock bed lies across a river. Erosion is retarded over it but the adjacent softer bed is eroded faster. As a result, a scrap and a waterfall develop there. When the flow of river is resisted by rocks of uneven slope, Rapids, Cuscades and Cataracts are formed.
- The middle or the plain course:
In plains transportation is the principal work of river than erosion and deposition. The landforms thus produced are:
- Alluvial cone or fan: On entering a plain the velocity of a river is suddenly reduced in consequence of abrupt fall of slope of land. Now the river cannot carry the coarser materials (sand, pebble, bolder) any more and deposits them at the foot of the mountain in the form of a cone or fan called alluvial cone.
- Wide ‘V’ shaped valley: In plains river velocity decreases and so decreases down cutting of a river; lateral erosion increases in proportion to vertical erosion resulting in the widening of the ‘V’ of the valley.
- Flood plain: The valley side slope being gentle a river gets the opportunity of forming larger meanders. Shifting meanders undercut the sloping valley sides and form flat land in their vicinity. Sediments are deposited over them at times of flood. Such a plain bordering a river is called a flood plain.
- Natural levee: During a flood the river bank is raised a little by silt deposition which reaches maximum there. When the flood subsides, the silt remains. Repeated floods raise the height of the bank sufficient to form an embankment like feature called natural levee.
- Ox-bow Lake: Over the flood plain a river takes ox-bow like meanders. At that time river current directly strikes inner edges of the two ends of a meander. Thus the ends of a meander are eroded at a faster rate and approach one another. At one time the river cut across the narrow neck of land between the meander ends which are gradually silted up. The abandoned channel is left over as an ox-bow lake.
- The delta or lower course:
The principal function of a river is deposition in this course. Major landforms in the course are:
- Delta: The velocity of a river is greatly reduced where it meets the sea or the lake. Moreover, it carries with it an excess of silt which is deposited on the river, the sea or the lake. In course of time a fan shaped alluvial tract, called delta, forms at the mouth of a river.
Deltas are found at the mouth of almost all big rivers; the Ganges, the Godavari, the Krishna and the Cauveri have well formed delta. The combined delta of the Ganges and the Brahmaputra is perhaps the largest delta of the world, but if the velocity of a river at its mouth or estuary and ocean current there remains sufficiently strong, deposition of silt cannot take place at the river mouth to form delta. For example the small west flowing rivers of the Peninsular India do not form deltas at their mouths.
- Natural levee, and
- Ox-bow lakes are also found over delta and are formed in manners that have been described under plain course of a river.
