Features formed by a river in the upper course
This is also known as river beheading or river piracy. Its development is dependant on the different rate of headward erosion (back-cutting) into a divide. For example – if one side of the divide has more gradient or receives more precipitation than the other, the process given below will follow.
Stream A will cut back more rapidly than stream B. Its higher erosive power will result in enlarging its basin at the expense of weaker stream.
Stream A will break through the divide and capture stream B.
The bend at which the piracy or capturing occurred is called elbow of capture. The beheaded stream is called the misfit.
The valley below the elbow is known as a wind gap and are often used a road or rail route.
Example – The Irrawaddy river captures the upper Sittang region in Burma.
In Northumberland, England, the Blyth and the Wansbeck are beheaded by the North Tyne.
Rapids, waterfalls and cataracts
Although these features can be seen in other parts of the river course too but these are most prominently seen in the upper part of the river course due to abrupt and frequent changes in gradient. The resistance is unequal due to the presence of hard and soft rock. The outcrop of a band of hard rock may cause a river to fall down or jump stream leading to the formation of rapids. The smaller falls of greater dimension are called cataracts. You will find five cataracts along the river Nile that hinder the smooth navigation. The sudden fall of the river from some height forms waterfall. These can be formed in many ways:
- When a bar of resistant rocks lies transversely across a river valley. Example – Niagra falls in the U.S.A. It is about 162 m in height. Also, Kaieteur falls in Guyana which around 825 high.
- When fault line scarp formed by faulting lies across the river. Example – River Zambezi forms the Victoria falls of 360 feet.
- When water plunges down the edge of a plateau. Example – River Congo which leaps for 900 feet through a series of more than thirty rapids as Livingstone Falls.
Features formed by a river in the middle course
A winding course is developed when water moves down the slope under the force of gravity. The water does not move in a straight direction for long distances. The irregularities of the ground force the river to swing in loops, forming meanders. Meander is a term derived from the winding River Meanderez in Asia Minor.
River-cliffs and river slip off slopes
To understand the mechanism look at the diagram and understand the steps:
- When the flow of the water PQ enters the bend of the river, it dashes straight into Q, eroding the outer bank into a steep river cliff at Q.
- The water piles up on the outside of the bend due to the centrifugal force.
- A current RS at the bottom moves in cork-screw motion and is hurled back into the midstream and the inner bank.
- At S, deposition of shingle takes place due to the gentle slip-off slope.
- Consequently, continuous erosion occurs on the outer bank while continuous deposition takes place in the inner bank.
As the stream flows on, the meanders migrate progressively outwards with the interlocking spurs alternating with undercut slopes. At this stage, meanders in the middle course are only the beginning of the downstream swing as bends are restricted by interlocking spurs. In the lower course, the loops are enlarged across the level plain and meanders are fully developed.
Features of the lower plains
Large quantities of sediments are carried by rivers by rivers. These sediments are spread over the low lying area during the sporadic or annual floods. This leads to gradual building of flood plain by deposition of layers of sediments every year. The bed of the river raised by accumulation and deposition of sediments on the sides. The sideways accumulation causes the banks to get raised and are called levees. When the water volume keeps increasing, it keeps coming close to the top of levees
To minimize the risk of floods, the erection of artificial embankments is done on the natural levees. But, this is not the permanent solution as this allows the river to rise further.
When they no longer tolerate the pressure of flood water, the banks burst, damaging property and drowning thousands. These kinds of floods are normally seen in the Yangtze Kiang, Po, Mississipi, Ganga plain. Hwang Ho, the river in China is known for floods and is also known as China’s sorrow.
These are also known as cut-offs or bayous in the Mississippi basin. The meander becomes pronounced in the lower course of the river. The outside bend or concave bank is so rapidly eroded that the river becomes almost a complete circle. Ultimately, the river cuts through the narrow neck of the loop, abandoning an ox-bow lake or Mortlake (dead lake). The river then flows straight. The ox-bow lake later declines into a swamp through succeeding floods that may silt up the lake.
The material which has not yet been dropped by the river on its course is now dropped or accumulated on the mouth of the river forming a fan-shaped alluvial area called delta. There are several kinds of delta –
- Mississipi – Bird’s foot delta, with several branches reaching the Gulf of Mexico
- The Nile, Mekong and Ganga – Fan shaped arcuate deltas with several distributaries
- The Vistula, Ob and Amazon – The deltas are partly submerged in coastal waters and form estuaries.
- Ebro of Spain – tooth like projections at the mouth are known as capsulate deltas.
Conditions favourable for the formation of deltas
- Lateral erosion and vertical erosion in the upper course of the river to provide extensive sediments to be eventually deposited as deltas.
- The coast should be sheltered and preferably without tides.
- The sea should be shallow near the delta region or else the load will disappear in the deep water.
- There should not be the presence of large lakes in the river course to filter off the sediments.
- The currents should be weak and no strong current should run at right angles to the river mouth as it can wash the sediment away.