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.
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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