The Course Of A River
YOUTH/UPPER/MOUNTAIN COURSE:
- This begins at the source of the river near the water shed, which is probably the crest of a mountain range. The river is very swift as it descends the steep slopes, and the predominant action of the river is vertical corrasion.
- The valley developed is thus deep, narrow and distinctively V-shaped.
- Down- cutting takes place so rapidly that lateral corrasion cannot keep pace.
- In some cases where the rocks are very resistant, the valley is so narrow and the sides are so steep that gorges are formed e.g. the Indus Gorge in Kashmir.
- In arid regions, where there is little rainfall to widen the valley sides, and the river cuts deep into the valley-floor, precipitous valleys called canyons are formed, e.g. the Bryce Canyon, Utah, U.S.A.
MATURE/MIDDLE/VALLEY COURSE:
- In the middle course, lateral corrasion tends to replace vertical corrasion.
- 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. The work of the river is predominantly transportation with some deposition.
- The river’s treble task of valley-cutting, bed-smoothing and debris-removal are being carried out in a more tranquil manner than in the mountain course though the velocity does not decrease.
OLD/LOWER/PLAIN COURSE:
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The river moving downstream across a broad, level plain is heavy with debris brought down from the upper course.
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Vertical corrasion has almost ceased though lateral corrasion still goes on to erode its banks further.
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The work of the river is mainly deposition.
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The volume of water is greatly swelled by the additional tributaries that join the main stream.