Development of River Valley
The erosional and depositional land features produced and modified by the action of running water may be better understood if we note the stages through which a stream passes from its source to its mouth. The source of a river may lie in a mountainous region and the mouth may meet the sea or lake. The whole path followed by a river is called its course or its valley.
The course of a river is divided into three sections –
- (i) The upper course or the stage of youth
- (ii) The middle course or the stage of maturity
- (iii) The lower course or the stage of old age.
THE UPPER COURSE
The upper or mountain course begins from source of the river in hilly or mountainous areas.
- The river tumbles down the steep slopes and as a result its velocity and eroding power are at their maximum.
- Consequently valley deepening assumes its greatest importance at this stage.
- Normally, weathering also plays its part on the new surfaces exposed along the banks of the stream.
- The weathered rock material is carried into the stream partly through the action of gravity and partly by rain water flowing into the river.
- Weathering helps in widening a valley at the top giving it a typical ‘V’ shaped cross section. Such valleys are known as ‘V’ shaped valleys.
If the bed rock is hard and resistant, the widening of the valley at its top may not take place and the down cutting process of a vigorous river may lead to the formation of a gorge i.e. a river valley with almost vertical sides.
In India, deep gorges have been cut by the Brahmaputra and the Indus in the Himalayas. Deep gorges also develop in limestone regions and in rocks lying in dry climates. The narrow and very deep gorge or the canyon with vertical walls is also known as ‘I’ shaped valley.
A canyon is very deep gorge with steep sides running for hundreds of kilometres.
- Grand Canyon of the river Colorado in U.S.A.
Some of the more outstanding features that are developed in the upper course of a river include rapids, cataracts, cascades and waterfalls.
THE MIDDLE COURSE
In the middle course, lateral corrosion tends to replace vertical corrosion. Active erosion of the banks widens the ‘V’ shaped valley. The volume of water increases with the confluence of many tributaries and this increases the river’s load. Thus work of the river is predominantly transportation with some deposition.
Rivers which sweep down from steep mountain valleys to a comparatively level land drop their-loads of coarse sand and gravels as there is sudden decrease in velocity. The load deposited generally assumes a fan like shape, hence it is called an alluvial fan.
Sometimes several fans made by neighbouring streams often unite to form a continuous plain known as a piedmont alluvial plain, so called because it lies at the foot of the mountain.
In this section even minor obstacles force a river to swing in loops to go round the obstacles. These loops are called meanders, a term derived from the winding River Meanders in Turkey.
THE LOWER COURSE
The river moving downstream across a broad, level plain is heavy with debris brought down from the upper and middle courses.
Vertical corrosion has almost ceased, the lateral corrosion still goes on to erode its banks further. The work of the river is mainly deposition, building up its bed and forming an extensive flood plain. Many tributaries join the river and the volume of water increases, coarse materials are dropped and the fine silt is carried down towards the mouth of the river. Large sheets of material are deposited on the level bed and the river splits into a maze of channels. Such a stream is called a braided stream.
During annual floods large quantities of sediments are spread over the low lying adjacent areas. A layer of sediments is thus deposited during each flood gradually building up a fertile flood plain. A raised ridge of coarse material is formed along each bank of the river. Such ridges are called levees.
In the lower course of the river, meanders become much more pronounced. The outer bank or concave bank is so rapidly eroded that the meander becomes almost a complete circle. A time comes when the river cuts through the narrow neck of the loop. The meander, now cut of from the main stream, takes the form of an oxbow lake.
This lake gradually, turning into swamps disappears in course of time. Numerous such partially or fully filled oxbow lakes are marked at short distance from the present course of river like the Ganga.
Upon entering a lake or a sea, the river deposits all the load at its mouth giving rise to the formation of a delta. Delta is a triangular relief features with its apex pointing up stream and is marked as a fan-shaped area of fine alluvium.
The Greek letter (△) pronounced delta closely resembles the triangular delta of the river Nile. Some deltas are extremely large. The Ganga – Brahmaputra Delta is the largest delta in the world.
The following conditions favour the formation of deltas –
- active vertical and lateral erosion in the upper course of the river to supply large amount of sediments;
- tide-less, sheltered coast;
- shallow sea, adjoining the delta;
- no strong current at the river mouth which may wash away the sediments.
Due to the obstruction caused by the deposited alluvium, the river discharge its water through several channels which are called distributaries.
Some rivers emptying into sea have no deltas but instead they have the shape of a gradually widening mouth cutting deep inland. Such a mouth is called estuary. The formation of estuaries is due to the scouring action of tides and currents. But in most of the cases the original cause is the subsidence of the earth’s crust in the area of the outlet.
The two west flowing rivers of India, the Narmada and the Tapi do not form deltas. They form estuaries when they join the Arabian Sea.
Upper, middle and lower are the three courses into which a river valley is divided.
Bibliography : NIOS – Geography