(World Geography) Drainage System & Landforms - Courses of Rivers, Mountain Courses, Rejuvenated Stream & River Capture
1. The Upper or Mountain Course
The river is very swift as it descents the steep stopes, and the predominant action of the river is vertical corrosion. The valley developed is thus deep, narrow and distinctively V-shaped. Down cutting takes places so rapidly that lateral corrosion cannot keep pace. In some cases where the rocks are very resistant, the valley is so narrow and the sides are so steep that gorges are formed e.g. the Indus Gorge in Kashmir. In arid regions, where there is little rainfall to widen the valley sides, and the river cuts deep into the valley-floor precipitous valleys called canyons are formed, e.g. the Bryce canyon, Utah, U.S.A,
a) River capture: This is also know as river piracy or river beheading. Its development is dependent on the different rate of back-cutting (head ward erosion) into a divide.
b) Rapids, Cataracts and Waterfalls: They are most numerous in the mountain course where changes of gradient are more abrupt and also more frequent. Due to the unequal resistance of hard and soft rocks traversed by a river, the outcrop of a band of hard rock may cause river to ‘jump’ or ‘fall’ down-stream and Rapids are formed. Similar falls of greater dimensions are also referred to as Cataracts. Of which there are five along the Nile that interrupt smooth navigation.
When rivers plunge down in a sudden fall of some height, they are called Waterfalls. Their great force usually wears out a plunge pool beneath.
Waterfalls are formed in several ways. Glaciation also produces hanging valleys where tributary streams reach the main U-shaped valley below as waterfalls.
2. The Middle Course
In the Middle course, lateral corrosion tends to replace vertical corrosion. Active erosion of the banks widens the V-shaped valley. The volume of water increases with the confluence of many tributaries and this increases the rivers load. The work of the confluence of many transportation with some deposition.
a) Meanders: As water flowing under gravity seldom flow straight for any long distance, a winding course soon develops. This is know as river meander. In meanders due to centrifugal force the velocity of water is more on outside bend. This results in erosion and formation of river cliff. A bottom current is set that deposits the sediments on the other side. It is known as slip off slope.
Oxbow lakes: The lakes formed due to impounding of water in abandoned meander loop accentuated due to lateral erosion, the meander loops become circular and come closer. During floods river straightens its course and after receding of flood meander loops are abandoned to form ox-bow lakes. It is to be noted that ox-bow lakes occur both due to erosion and deposition both.
b) Alluvial fans and cones: Alluvial fans are formed are formed due to accumulation of sediments at the base of the foothills when there is abrupt decrease in channel gradient. Due to decreases, sediment load becomes too heavy for the streams to be carried over gentler gradients and gets dumped and spread as a broad low to high cone shaped deposit called alluvial fan. Alluvial cones have steeper slope (15o) than fans (5o). The size of the sediments decreases outward from the apex (towards the hill) of the fans toward the distal margins.
3. The Lower or Plain Course
Vertical corrosion has almost ceased though lateral corrosion still goes on to erode its banks further. The work of the river is mainly deposition, building up its bed and forming extensive flood plains.
Flood Plain: Rivers in their lower course carry large quantities of sediments. During annual or sporadic 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 flood plain. When the rivers flow normally its bed is raised through the accumulation of deposits and material is also dropped on the sides forming raised banks called levees.
Deltas: A river delta is triangular shaped depositional feature formed at the mouth of a river debouching either in lake or sea . when a stream enters a standing or slower moving body of water such as a lake or ocean, there is a sudden decrease in velocity and the stream deposits its sediment in a deposit called a delta (reduction in flow velocity decreases load carrying capacity). Deltas build outward from the coastline, but will only survive if the ocean currents are not strong enough to remove the sediment. Deltas develop only at those river mouths where the fluvial sediment supply is high, the underwater topography does not drop too sharply i.e. geologically stable area, and waves, currents, and tides cannot transport away all the sediments delivered by the river. The size of delta also depends on the size of drainage basin, rock characteristics, vegetal cover, rate of erosion, amount of annual rainfall etc. Almost every river, whether small or large forms a delta.
Arcuate - It is a fan shaped delta where maximum deposition is towards the centre. These types of delta are composed of relatively coarse sediments. The Nile Delta is classic example of an arcuate delta as also Ganga, Indus, Niger, Hwang Ho (Yellow), Volga, Rhine, Danube, Godavari delta etc.
Bird’s Foot - Here the action of waves is weak and that of a river is strong, so an irregular-shaped delta forms that extends out into the water well beyond the local shoreline. Here the deposition is along the sides of distributaries. The Mississippi Delta, on the Gull of Mexico, is a bird’s foot delta.
Cuspate - A cuspate delta is tooth shaped. Cuspate
deltas form where a river drops sediment onto a straight shoreline with strong
waves that hit head on. The Ebro delta in Spain, Tiber Delta in Italy, Sao
Francisco river in Brazil etc.
Estuarine delta- This type of delta has a river that empties into a long, narrow estuary that eventually becomes filled with sediment (inside the coastline). Those months of the river are called estuaries which are submerged under marine water and sea waves and oceanic currents remove the sediments brought by the rivers e.g. Narmada and Tapi delta in India, Seine River of France, Vistula, Elb, Ob, Hudson delta etc.
BASE LEVEL & REJUVENATION
When the base level of a river (Base level is the lowest level to which downcutting by a river is possible. The ultimate base for any stream is the water body into which it flow – sea, lake, reservoir, dam, etc) deepens it enhances the erosion capacity of the main and tributary streams. This rejuvenates the life stage of river i.e. return it youthful stage of erosion. The capacity of river to erode its base is enhanced. Thus rejuvenation is the renewal of a river’s energy in to a relative fall in base level.
Causes of River Rejuvenation:
1) Upliftment of land,
2) Fall in sea level, and
3) River Capture.
Features of Rejuvenated Stream
1) Incised meanders: In the middle and lower course vertical corrosion replaces lateral corrosion 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 developed a deep valley with incised meanders. The best developed incised meanders are those of the River Colorado.
2) Knick points: Knick points appear on a rivers long profile and are, almost, like the steps down a river takes to reach the new base level. It represents the head of rejuvenation. They are usually associated with waterfalls.
3) River terraces: A Rejuvenated stream starts cutting the former flood plain with greater vigour leaving terraces on both sides.
The diversion (capture) of the headwaters of one river into a neighbouring river is called river capture or stream piracy. River capture occurs when one stream lengthens by headward erosion (backwards erosion at its source), cutting into the interfluve that divides it from another stream. Eventually, as its valley becomes larger and the interfluve smaller, the drainage divide between it and the other stream will shift, so it will eventually capture the water once destined for the other stream. River capture of youthful and mature rivers is a common feature of the regions of folded. The regions of folded rocks. A stream whose upper waters have been captured is said to be beheaded. The valley of captured river just below the point of capture, now being dry (having no water) are called wind gaps or dry gaps.
At the point of diversion there is usually a marked bend, known as elbow of capture. The river which has lost its headwaters will be much reduced in volume, and therefore too small for its existing valley, hence it is called a misfit stream. Such a captured river may be so much smaller that its source will now be some distance below the point of capture. In exceptional cases, as states above, the valley remains dry.