Complete information on concept of River Morphology

A river is a natural channel about a few kilometers or more long carrying water along the slope of land surface. There is no limit to the size. Such terms as brook or rivulet stand for small streams. The Indian word ‘nala’ or ‘nulla’ is probably more distinct and is rarely mistaken for river. Although the word ‘nadi’ is loosely spoken for rivers of any size it occasionally stands for smaller streams, and the word ‘daria’ stands for larger streams.

The morphology of a river can be viewed conveniently by considering its long profile and cross profile. Long profile is the section or line, which can be obtained by plotting the axial line of the channel from source to mouth. Such a section will indicate the slopes in the different reaches of the channel, the various major or minor knicks, marked by cataracts, waterfalls or rapids. It will also show any lakes that may occur along the path of the river. The details of the long profile will show potholes if any.

If we consider rivers vacated of water the floor of the channel will expose all the details of the long profile. The upper surface of the stream or river water in case of perennial rivers will also reflect the general character of the long profile.

The other characteristics of the river morphology can be seen through cross-profiles in the different sections of the river. Such profiles will show the characteristics of the banks, the natural levees, the meanders, the width and depth of channels, the slopes of the banks or valley flanks, the braiding of the streams, river terraces, alluvial cones, fans, and deltas, the nature of the channel floor including potholes, etc.

The various features of river morphology mentioned above are associated with certain physical process.

Long Profile of River:

As water flows along the channel of a river, its long profile will be determined by several factors, which include geology, structure, original slope, discharge, etc.

In the source region, the limited catchment supplies little volume of water whence erosion is limited. As the river precedes downstream the volume increases and erosion gains momentum. Nearer the mouth, however, because of low gradient deposition exceeds erosion. Because of such varying degrees of erosion, the long profile becomes concave.

Such concavity, however, is negligible and imperceptible in the case of streams flowing through dead level plain, e.g., the streams originating within the Ganga Plain like the Gomati, the Sai, the Daha, etc. The concavity is well marked where the course of the river is from a mountain or upland through a plain or peneplain to the sea level, e.g., the Ganga and other Himalayan rivers. If an imaginary uniform slope were superposed on the present concave long profile, the concavity will be located mainly in the mountain or upland section the lower part of the long profile being relatively flat.

“The long profile of the Ganga and all the major Himalayan tributaries (on an exaggerated vertical scale, which is essential for emphasizing the features of the long profile) is L-shaped with a relatively steep section representing the Himalayan part of the course where there is a drop up to 6,000 meters within a distance of about 150 km and a nearly horizontal limb representing the plain section of the course where the drop is less than 300 meters over a distance of 1,800 km”. The vertical limb of L is higher and steeper in the case of larger rivers like the Ganga, the Ghaghara, the Gandak and the Kosi. It is shorter and gentler in the case of the shorter tributaries of the Ganga, e.g., the Ramganga, the Baghmati, the Kamla and the Mahananda.

The long profile of the Peninsular Rivers is not ‘L’ shaped. It is ‘easy-chair’ type. It is illustrated by such rivers as the Chambal, the Betwa, the Ken, the Tons, the Son, the Punpun, the Chandan, etc. which have a relatively steep profile across the Central Indian Foreland and Chotanagpur and a gentler almost horizontal limb across the Ganga Plain.

However, the terms ‘L’ and ‘easy-chair’ types are descriptive expressions to bring out the contrast in the long profiles of the mountain and plain sections. The actual amount of slopes is much humbler. Thus, the slope of the Himalayan sections of the streams is about 4% whereas their plain-sections have a gradient of about 0.01%. In the case of the peninsular streams the slope is about .5% and in the plains about 0.05%.

In other words, the Himalayan gradient is about ten to twenty times the peninsular gradient of the Gangetic streams. However, another significant fact in the long profile of the Himalayan streams is the absence of remarkable knick points despite the relatively steep gradient. This is indicated in the general absence of waterfalls along the Himalayan streams.

In the higher Himalayas, however, where glaciated hanging valleys are common there are a number of high waterfalls, c. 100 metres, as on the Mandakini and the Pindar, tributaries of the Ganga. As contrasted with this the Central Indian Foreland particularly the Vindhyan scarps and Kaimur Plateau and Chotanagpur scarps are associated with remarkable knicks so that a series of waterfalls, some as high as 60 metres, dot the northern margin of the Peninsular upland as the Gangetic tributaries descend the scarped edge of the 300 metre erosion surface.

Thus if there is a ‘fall line’ in the Ganga basin it is on its southern margin. The absence of knicks on the Himalayan margin due to the comparative softness of the young poorly-compacted, soft Siwalik sedimentaries, and the prominence of knicks on the southern edge of the Ganga basin is due to the resistance of the quartzitic sandstone of the Upper Vindhyans and gneisses and granites of Bundelkhand and Chotanagpur. Both the northern and southern edges of the Ganga plain underwent Pleistocene uplift but while such movement sharpened the edges and knicks in the south, in the north it only meant a deeper incision into the softer sub-Himalayan rocks.

But although there is no prominent fall-line in the north there are minor knicks and this ‘knick-line’ is indicated by the headworks across the Kosi, Gandak, Sarda and Ganga in the Shiwalik foothills. The relatively small streams that descend the basaltic fault-scarp of the Western Ghats towards the Arabian sea have long profile similar to the Himalayan rivers but on a smaller scale. Because they descend a stupendous scarp of relatively recent origin, they have waterfalls along what may be termed as Western Ghats fall line.

Waterfalls:

The long profile of rivers may be marked by waterfalls whose causes may be various. We can make artificial model channels with breaks in the long profile. These breaks may consist of materials of different hardness, e.g., clay, sandrock, shale, sandstone, and quartzite. The fall will endure according to the resistance of the rock over which water falls.

Another factor determining the nature of the fall will be the height of the knick. If the knick rock is underlain by softer material, the latter will collapse rapidly ensuring the verticality of the fall. The durability of the fall will vary directly with the horizontal extent of the hard bed of rock.

Falls Due To Earth Movement:

According to Wadia, the waterfalls of Peninsular India, e.g., Dhurandhar (9 metres) or Bhera Ghat falls on the Narmada, the Yenna falls in the Mahabaleshwar hills (180 meters high), Gokak falls in Belgaum district (54 metres high), Gersoppa fall on the Sharavati in North Kanara droping over the scarps of the Western Ghats by 260 meters and Sivasamudram fall on the Cauvery (90 meters high) in Mysore, are due to ‘minor disturbances in late geological age’. Such disturbances or earth movements are invoked because the Peninsular uplands are very ancient, Pre-Cambrian land areas and the long-profiles or rivers should have become free from irregularities because of their long age.

The northern edges of peninsular uplands (or for that matter the whole of Peninsular India) where they adjoin the Ganga plain are believed to have undergone uplift as side effects of the Himalayan orogeny particularly during late Tertiary. This brought about the various scarps, e.g., the Vindhyan and Kaimur scarps, the scarps of Chotanagpur, those of Western Ghats, Mysore Plateau and Orissa highlands. As the streams descend they are marked by waterfalls. However, mere uplift cannot account for the waterfalls. If this were so the long profiles of all the rivers of the Western Ghats and other scarps and knick-lines should have been marked by waterfalls. This is not so.

Lithology an Important Cause:

The local lithological and structural characteristics also are important contributory factors. These are important anywhere as also in the few falls that occur in Himalayas, Bhutan, Nepal, etc.

According to Holmes, the Gersoppa or Jog falls belong to the type where ‘rivers pass from uplifted highlands of metamorphic and massive igneous rocks to a coastal plain. The rivers drop either in hard basaltic ledges as in the Deccan lava region or Rajmahal hills or quartzitic sandstones as in Kaimur Plateau or gneisses and granites indurated by silica injections. The examples of such waterfalls on the knick-line of Kaimur Plateau in Bihar are Chhanpathar (90 m) on the Karamnasa, and Khadar Khoh (90 m) on the Durgaoti. Some of the waterfalls on the knick-lines formed by the scarps of Chotanagpur are Hundru falls (75 m) along the Subarnarekha, the Dasam (40 m) on the Kanchi, the Sadni falls (60 m) on the Sankh, and the Kakolat (50 m) 16 km south of Nawadah due to hanging valleys.

Falls due to hanging valleys:

The Indian waterfalls have not been studied in detail. If it were done, a vast variety of controlling factors will be brought to light. The Rajrappa waterfall in Chotanagpur where the Bhera nadi, a tributary of the Damodar, hangs above the major stream was noted by Ahmad. The site presents an awful spectacle of jagged surface of granite gneiss with prominent joints all dipping at an angle of about 55°. The predominantly rocky and uneven ground in the middle of which the Damodar stream occupies a narrow entrenched course stretches between well-marked banks about 150 metres apart. The depth of the stream bed from the immediate shoulders is about 15 metres while the drop of the Rajrappa waterfalls is about 10 m (Dec.) During the rains particularly after heavy showers the water spreads over the wider valley. The uplift of the region during Pleistocene and probably in still later times has helped the larger volume of the main stream, i.e., the Damodar to carve out a deeper channel leading to the hanging nature of the tributary, the Bhera nadi. The local gradient of the two streams being nearly equal, the larger volume of water in the Damodar makes the difference in the depth of the channels.

Hundru Waterfalls:

The most important waterfalls in Chotanagpur the Hundru falls on the Subarnarekha. The structure is granite-gneiss. The main drop is on a nearly vertical ‘scarp-face-like ledge. It could be interpreted as due to local faulting because as believed by J.A. Dunn all the scarps in Chotanagpur are due to successive Tertiary faulting. The Hundru falls are located in a scarp zone. Alternatively the ‘fall-face’ could be due to a large joint exposed by the falling away of the downstream portion.

It is believed by some that the waterfalls in the tropical regions are more durable because here the hot and humid climate favouring strong chemical weathering reduces the erosion tools (larger fragments and larger sediments) to finer sediments whence the falls persist. In the temperate and colder areas, the erosion tools are large and obliterate the falls more quickly. A detailed survey of all the waterfalls in the world alone may support or contradict this view.

The break of slopes at the falls by itself ensures the formation of larger fragments in the stream. This is one of the main causes of fall reduction and obliteration.

Definition of Waterfalls:

A waterfall is a vertical drop of water of considerable magnitude both in volume and height. Cataract is a synonymous expression. Rapids are of smaller magnitude. Whereas the dip of rocks at the discordance in the waterfalls may be upstream, it may generally be downstream in the case of rapids. The rapids, which are common in Chotanagpur, may be due to a sudden steepening of slope. This steepening may be caused by the quicker erosion of softer elements below harder rocks at the lower end of which water tumbles (as on the Bhusur nadi 300 m, west of Hinoo Bridge in Ranchi city).