Thursday, 31 December 2015

River Rejuvenation

The earth's crust is far from stable and it is not surprising that, in the course of a river's development, parts may be uplifted or depressed, giving rise to certain characteristics features associated with rejuvenation, i.e. being young again.
A negative movement occurs when there is an uplift of land or a fall in sea level. This will steepen the slope so that active down-cutting is renewed. A fall in sea level leaves the flood-plain at an increased altitude above the sea level. The river with its renewed vigour cuts into the former flood-plain, leaving behind terraces on both sides of the river. There is also a break in the graded profile of the river, often marked by a series of rapids. This point where the old and rejuvenated profile meet is called the Knick Point or Rejuvenated Head.
If rejuvenation occurs in the upper-course, the river valleys are deepened and steep-sided Gorges are formed. In the middle and lower course vertical corrasion replaces lateral corrasion and the existing meanders are vertically eroded by the rejuvenated stream. A distinct new inner trench is cut in the old valley, and the river develops a deep valley with entrenched or incised meanders. The best developed incised meanders are those of the River Colorado, U.S.A., where the uplift of 7,000 feet in the tertiary period renewed down-cutting to a fantastic depth. In some parts of the Grand canyon, the depth is almost a mile. It is 10 miles wide at the top and 300 miles long. Other examples are the River Moselle in Germany, the River Wear at Durham, England and the Wye Valley, Monmouthshire.
A positive movement occurs when there is a depression of land or a rise in sea level. This submerge the lands along the coast, `drown' the valleys and weaken the erosive power of the river. The flow is checked and large quantities of sediment will be dropped. The lower course of the river may be partly in the sea and features of deposition are shifted upwards to the middle course. The upper course is little affected when there is a rise in sea level. In many areas where the sea has risen this was probably caused by the release of water locked up in the ice masses during the Quaternary Ice Ages.
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