THE LANDFORMS MADE BY RUNNING WATER 

1. The development of the river system

The mechanism of humid erosion

The process of river action

• Materials in solution: These are minerals that are dissolved in water.
• Materials in suspension:  Sand, silt and mud are carried along and suspended in the water as the stream flows.
• The traction load: Pebbles, stones, rocks and blunders are rolled along the river bed and are called the traction load. More than 200 tons and 50 tons of suspension and solution materials are carried by rivers every year. The Mississippi River removes more than two million eroded materials into the Gulf of Mexico daily. The ability of the river depends on the velocity of flow, volume of water and size, shape and weight of the load.

2. River erosion and transportation

• Corrosion and abrasion: This is the mechanical grinding of the river's traction load against the banks and bed of the load. The rock fragments are hurdled against the sides of the river and rolled along the bottom of the river. Corrosion takes place in two ways.
• Lateral corrosion: It is the sideways erosion that forms v-shaped valleys. Vertical corrosion; is the downward action, which deepens the river channel.
• Corrosion or solution: It is the chemical or solvent action of water on soluble. For example, calcium carbonate in the limestone is easily dissolved and removed in solution.
• Hydraulic action; is the mechanical loosening and sweeping away of materials by the river water itself. Some of the water splashes against the riverbanks and surges into cracks and crevices. This helps to disintegrate the rocks. The water also undermines the softer rocks with which it comes into contact.
• Attribution: The wear and tear of the transported materials themselves roll and collide with one another.

2.1. The course of the river

2.1.1. The upper or mountain course

This begins at the source of the river near the watershed. This is probably the crest of a mountain range. The valley developed in deep, narrow and distractively v-shaped. Resistant rocks, narrow valleys and steep sides together formed the Gorges. Where there is little rainfall to widen the valley sides the river cuts deep into the valley floors and the precipitous valleys form the canyons. The following are the features often developed by the upper or mountain course.

River capture: Its development is dependent on the different rates of bank cutting [head ward erosion] into a divide. It is also known as the river beheading or river privacy. One side of the divide receives more perception than the other. The bend at which the privacy occurred is termed the elbow of capture. The valley at the elbow is called the wind gap. The beheaded stream is known as a misfit.

Rapids, cataracts and waterfalls

• These are occurring in the mountain course where the changes of the gradients are more abrupt and frequent.
• Due to the unequal resistance of the hard and soft rocks transverse by a river, the outcrop of a band of hard rock may cause a jump or fall downstream. Rapids are formed.
• Similar falls of greater size are referred to as cataracts.
• When the rivers jump sudden fall of some height is said to be waterfalls.
• The greater force usually wears out a plunge pool beneath.
• Waterfall forms in several ways.
• A bar of resistance rock lies transversely across a river valley, e.g. Niagara Falls.
• When a fault line scrap caused by faulting lays across the river, e.g. Victoria Falls.
• When the water plunges down the edge of a plateau-like river Congo which leaps for 900 feet through a series of more than 30 rapids as living stone falls.

2.1.2. The middle or valley course

Lateral corrosion tends to replace vertical corrosion. Active erosion of the banks widens the v-shaped valley. The volumes of water and river load increase with the joining of many tributaries, the work of transportation with some deposition.

• Meanders: Water flowing under gravity once in a while flows straight for any long distance, and a winding course soon develops river meander.
• River cliffs and slipping of slopes: Due to the centrifugal force, the velocity of water is more on the outside bend, resulting in erosion and the formation of river cliffs. A bottom current is set that deposits the sediments on the other side known as the slip-off slope. The outer bank has continual erosion and the inner bank has continual deposition.
• Interlocking spurs: As the stream flows on the meanders migrate progressively outwards with interlocking spurs alternating with the undercut slopes. The meanders in the middle course are only the beginning of the downstream swings, for bends are restricted by the interlocking spurs. On the lower course, the loops are enlarged across the level plains and meanders are fully developed.

2.1.3. The lower or plain course

The river moves downstream with heavy debris brought down across broad from the upper course. Vertical erosion has almost ended and lateral erosion still goes and erodes its banks. The work of the river is mainly deposition, building up beds and formation of vast flood plains. A large sheet of materials deposited on the level plain and May splitting of rivers into many complicated channels is called a Braded stream.

Floodplain: Rivers in their lower course carry large quantities of sediments. During annual floods, these materials are spread over the low-lying adjacent areas. A layer of sediment is thus deposited during each flood, gradually building up a fertile floodplain. The accumulation of deposits and materials that are also dropped on the sides of the banks is called levees. In an attempt to minimize the risk of floods artificial embankments are enacted on the natural levees.

 Oxbow lakes: These lakes are also known as cut-offs or bayous in the Mississippi basin. A river cuts through a narrow neck of the loop, abandoning an ox-bow lake.

Delta: A delta is an area of low, flat land shaped like a triangle, where a river splits and spreads out into several branches before entering the sea. 

Several types of deltas

• The Mississippi has a bird’s foot delta with several main branches in bird’s foot shape extending to the Gulf of Mexico.
• The Nile, gangs and Mekong have a fan-shaped arcuate delta with numerous distributaries.
• The Amazon, Ob and Vistula have their deltas partly submerged in coastal waters to form an estuarine delta.
• The Ebro of Spain has a tooth-like projection in their mouths. These are known as cuspate deltas.

The following summarizes the conditions favourable for the formation of deltas.

1. Active vertical and lateral erosion in the upper course of the river provides extensive sediments to be eventually deposited as deltas.
2. The coast should be sheltered preferably tideless.
3. The sea adjoining the delta should be shallow or else the load will disappear in the deep waters.
4. There should be no large lakes in the river course to filter the sediments.
5. There should be no wrong current running at the right angle to the river mouth, washing away the sediments.

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 characteristic features. A negative movement occurs when there is an uplift of the 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 floodplain at an increased altitude above the sea level. A positive movement occurs when there is a depression of land or the rise of the sea level.

3. The human aspects of rivers

In many countries, rivers form the chief high ways of commerce and trade. All the rivers undertake three major interrelated activities erosion, transportation and deposition.

• Disadvantages of rivers:  Rapids and waterfalls interrupt the navigation of the river.  A large amount of deposits of sediments in the lower course of the river splits up and prevents of anchoring of large ships near shore. These can be overcome by the construction of artificial harbours.
• Advantages of rivers: In the upper course, rivers with steep gorges and waterfalls provide natural sites for the generation of hydroelectric power, leading to the establishment of different industries. Dams were constructed across the rivers to hold back the water for drinking, domestic, irrigation and industrial purposes.