River is fresh water
that flows from its source to or empties into a sea, lake or larger river.
These rivers lift various materials and continue to be deposited. As a result
of this process a river valley is formed, which is a lower surface form than
other parts produced by erosion of water or river erosion.
River erosion or can be
called ablation (erosion by water) is the event of moving a rock or soil caused
by continuous river water flowing. Basic erosion is erosion that occurs at the
bottom of the river which will cause the riverbed to become deeper. Edge
erosion is erosion that occurs on the banks of the river which will later cause
widening on each side of the river.
River valley in the Grand Canyon
Object Souce:
There are several
stages that can explain the erosion by river water flow:
1. Flowing river water creates friction on the
ground. And the size of the friction that occurs is influenced by the size of
the flowing water.
2. Friction is greater if the speed and amount of
water are also getting bigger.
3. The water speed is also greater if the gradient
or slope of the land is getting bigger.
4. Friction between water and solid objects that are
transported by water with soil or rocks at the bottom of this can cause
erosion.
The process of river
valley formation takes place through three processes, namely deepening,
widening, and elongation.
a. Deepening of
the river valley
In the upper
reaches of the river, the erosion process is very strong because the height
difference is very high. High flow speeds cause the process of erosion and
transportation to work more dominantly. Its erosion power works by mashing and
grinding the riverbed called hydraulic erosion. Then followed by mechanical
erosion, the rock debris carried by heavy flow also erodes and accelerates the
deepening of the channel. In addition, chemical erosion also occurs, namely the
dissolution process and acid reaction to the bottom and edge of the river
channel
b. Widening of
the river valley
In flat areas,
the most dominant erosion process is lateral erosion due to the slow speed of
flowing water. Its nature widens the river valley channels. There is also the
process of adding deposits from material wasting (mass wasting) on the slopes
above which is called the agradation process which helps accelerate the
widening of the river valley.
c. Elongation of
the river valley
This event
occurred because of a decrease in sea level, so that land increased. And
because of the growth of the delta, the land face also increases. Delta is
sediments in river mouths originating from land, sand, small rocks, and various
materials carried by the river flow to the edge of the sea.
Delta Formation Scheme
Object Source: https://www.youtube.com/watch?v=XINpPb8XwJU
If the amount of damage
carried by the river water flow is very much and can not be carried away by
water again, then the results of weathering carried by the water will
accumulate somewhere. Here the erosion stops, but is followed by erosion to the
side. So a river that initially flows rather straightly turns and forms
meanders. Because of the meanders, the river valleys became wider.
Meander
Object Source: https://www.youtube.com/watch?v=4qKS_Nk7UmY
The development of a
river valley shows its age. Age here is the relative age based on the
appearance of the shape of the valley that occurs in several stages (stadium).
At the beginning stage, the river gradient is still large so that the vertical
erosion power is large. At this stage the original plain has just been formed.
This can be caused by the removal of the seafloor to the surface or eruption of
volcanoes which produce very much sedimentation so that new land surface is
formed. In some places there are beginnings of rivers with small valleys. So at
this stage the surrounding area is still a between streams and erosion is just
beginning to occur.
The river flow is
divided into two, namely the young and adult stadium rivers.
The characteristics of
the young stadium are as follows.
1. Cross section of a V-shaped valley. This occurs
because of the strong vertical abrasion due to the large gradient.
2. Many rivers still have temporary base erosion.
3. Transport of rivers is large.
4. The width at the bottom of the valley is equal to
the width of the river channel.
5. The bottom of the valley is not even.
The characteristics of
the adult stadium are as follows.
1. The river gradient is smaller.
2. Lateral or side erosion. While practical vertical
erosion has not occurred.
3. Has undergone riverbed leveling.4. The valley
forms the letter U, whose width exceeds its depth.
5. At the valley floor there is a flood plain and on
a plain flood the river forms meanders.
6. There is no basic erosion so that the river
valley bottom is evenly distributed.
Reference:
·
Wardiyatmoko, K.
2004. Geografi SMA Jilid 1 untuk Kelas X
Kurikulum 2004 Berbasis Kompetensi. Jakarta: Erlangga.
·
Rahayu, Saptanti
and friends. 2009. Nuansa Geografi 1:
untuk SMA/MA Kelas X. Jakarta: Pusat Perbukuan Departemen Pendidikan
Nasional
·
https://ilmugeografi.com/ilmu-bumi/hidrologi/pengikisan-oleh-air
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