Tuesday, December 31, 2013

How the Waterfalls are formed? With special reference to Ranchi plateau in Jharkhand State of India.



The waterfalls of Ranchi plateau are due to disturbances in late geological age.
By


Dr. Nitish Priyadarshi

Fig.1 Johna falls is the example of waterfalls resulting from upliftment. 
Fig.2 Dasam falls formed due to upliftment.



 Fig. 3 Hundru falls is the example of knickpoint falls.

A waterfall is a place where water flows over a vertical drop in the course of a stream or river. Waterfalls are also called cascades. Waterfalls are commonly formed when a river is young. At these times the channel is often narrow and deep. When the river courses over resistant bedrock, erosion happens slowly, while downstream the erosion occurs more rapidly. As the watercourse increases its velocity at the edge of the waterfall, it plucks material from the riverbed. Whirlpools created in the turbulence as well as sand and stones carried by the watercourse increase the erosion capacity. This causes the waterfall to carve deeper into the bed and to recede upstream. Often over time, the waterfall will recede back to form a canyon or gorge downstream as it recedes upstream, and it will carve deeper into the ridge above it.


The process of erosion, the wearing away of earth, plays an important part in the formation of waterfalls. Waterfalls themselves also contribute to erosion.

Often, waterfalls form as streams flow from soft rock to hard rock. This happens both laterally (as a stream flows across the earth) and vertically (as the stream drops in a waterfall). In both cases, the soft rock erodes, leaving a hard ledge over which the stream falls.

Erosion is just one process that can form waterfalls. A waterfall may form across a fault, or crack in the Earth’s surface. An earthquake, landslide, glacier, or volcano may also disrupt stream beds and help create waterfalls. 

Running water always erodes rock, but some rocks are more resistant than others. So a waterfall occurs when geological forces have produced either a sudden change in rock types or a steepening of a gradient where a stream is flowing. With a break in elevation, a stream or river becomes a waterfall.

Geology of Chota Nagpur Plateau:

The Chota Nagpur Plateau is a plateau in eastern India, which covers much of Jharkhand state as well as adjacent parts of Odisha, West Bengal, Bihar and Chhattisgarh. The Indo-Gangetic plain lies to the north and east of the plateau, and the basin of the Mahanadi River lies to the south. The Chotanagpur Plateau (22°-25° 30'N and 83°47'-87° 50'E) covering an area of 87,239 sq. km includes Ranchi, Hazaribagh, Singhbhum, Dhanbad, Palamau, Santhal Parganas (Jharkhand) and Purulia dis­tricts (West Bengal). It is composed of Archaean granite and gneiss rocks with patches of Dharwar rocks (phyllite, mica-schists).

The Chota Nagpur Plateau of Jharkhand state is a continental plateau - an extensive area of land thrust above the general land. The plateau has been formed by continental uplift from forces acting deep inside the earth. The Gondwana substrates attest to the plateau's ancient origin. It is part of the Deccan Plate, which broke free from the southern continent during the Cretaceous to embark on a 50-million-year journey that was violently interrupted by the northern Eurasian continent. The northeastern part of the Deccan Plateau, where this ecoregion sits, was the first area of contact with Eurasia.

Chotanagpur consists of a series of plateaus standing at different levels of elevation; the highest general elevation of about 1100 m in the mid-west­ern portion known as the Pat lands. From here the land descends in all directions in a series of steps particularly towards the east until it merges gradu­ally with the Lower Ganga Plain. The sharp break in slope are marked by steep scarps where the rivers like Barakar, Damodar, Subamarekha north and south Koels have carved out deep gorges and water­falls. The most characteristic features of relief are revealed in the Hazaribag and Ranchi plateaus standing at same general elevation (600 m) but separated by the Damodar trough (Permo-Triassic trough fault).

Water falls of Ranchi plateau.

It is the largest part of the Chota Nagpur Plateau. The elevation of the plateau land in this part is about 700 metres (2,300 ft) above mean sea level. The general topography is undulating. The Ranchi plateau gradually slopes down towards south-east into the hilly and undulating region of Singhbhum (earlier Singhbhum district or what is now Kolhan division. The plateau is highly dissected. Damodar River originates here and flows through a rift valley. To the north it is separated from the Hazaribagh plateau by the Damodar trough.To the west is a group of plateaux called pat.

There are many waterfalls at the edges of Ranchi plateau where rivers coming from over the plateau surface form waterfalls when they descend through the precipitous escarpments of the plateau and enter the area of significantly lower height. The North Karo River has formed a 17 metres (56 ft) high Pheruaghaugh Falls at the southern margin of Ranchi plateau. Such falls are called scarp falls. Hundru Falls (75 m) on Subarnarekha River near Ranchi, Dassam Falls (39.62 m) on Kanchi River, east of Ranchi, Sadni Falls (60 m) on Sankh River (Ranchi plateau) are examples of scarp falls. Sometimes waterfalls of various dimensions are formed when tributary streams join the master stream from great height forming hanging valleys. At Rajrappa (10 m), the Bhera river coming over from the Ranchi plateau hangs above the Damodar River at its point of confluence with the latter. The Jonha Falls (25.9 m) is another example of this category of falls. In fact the Gunga River hangs over its master stream, Raru River (to the east of Ranchi city) and forms the said falls.

Geological formation of Waterfalls in Ranchi Plateau.

The waterfalls of Ranchi plateau are due to disturbances in late geological age. The Ranchi plateau (or the whole of Peninsular India) believed to have undergone uplift as the side effects of the Himalayan orogeny particularly during the late Tertiary. As the streams descend they are marked by waterfalls. The most important water falls in Ranchi Plateau are the Hundru falls on the Subarnarekha. The structure is granite-gneiss. The main drop is on a nearly vertical. It could be interpreted as due to local faulting because as believed all the scarps in Ranchi plateau are due to successive Tertiary faulting.

The Chotanagpur Plateau is made up mainly of Precambrian rocks but has witnessed uplifts synchronously with Himalayan uplift in the Cenozoic.

According to another concept falls of the plateau are formed due to sudden steeping 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 river 300 m., west of Hinoo bridge in Ranchi city).

Waterfalls of varying dimensions are formed due to upliftment of local nature in the courses of the rivers. These waterfalls are obliterated when the rivers regrade their longitudinal profiles. A series of waterfalls on the river along the junction of Palamau upland and the northern flat plain ( Palamau district Jharkhand) are said to have been formed due to orogin of escarpment caused by upliftment of southern Palamau during Tertiary period. Patam falls ( 45.72 m) and Datam falls (30.45 m) on Patam river ( in Bhandaria Anchal, Palamau, Jharkhand) are typical examples of such categories. The waterfalls on the eastern margin of Ranchi Plateau ( e.g. Hundru falls on Subarnarekha river, Dasam falls on Kanchi river, Johna or Gautamdhara falls on Gunga river etc.) are also quoted as the examples of waterfalls resulting from upliftment.

Some times, waterfalls of varying dimensions are formed when the tributary streams join their master streams from great height forming hanging valleys. In other words, hanging valley falls are formed when the level of the junction of the tributary streams is much higher than the level of the main valley of the master stream. The Rajrappa falls ( 10 m.) at the junction of the Bhera river and the receiving Damodar river (located to the north of Ranchi city) is a typical example of hanging valley waterfalls as the Bhera river after coming from over the Ranchi Plateau hangs above the Damodar river as its confluence with the latter. The Gautamdhara or Johna falls ( 25.9 m.) is another example of this category of falls. In fact, the Gunga river hangs above its master stream, Raru river, ( to the east of Ranchi city) and forms the said falls.

Some falls of Ranchi plateau comes under knick point falls. The breaks in channel gradient caused by rejuvenation are called knick points or heads of rejuvenation. These breaks in channel gradient or knickpoints denote sudden drops of elevation in the longitudinal profile of the rivers and allow the water to fall down vertically giving birth to waterfalls of varying dimensions. Hundru falls ( 76.67 m) on Subarnarekha river ( near Ranchi city), Johna or Gautamdhara falls at the confluence of Raru and Gunga rivers (to the east of Ranchi). Dasam falls ( 39.62 m and 15. 24 m ) on Kanchi river (east of Ranchi) are the examples of knickpoint falls.

Reference:

Ahmad, E. 1985. Geomorphology. Kalyani Publishers, New Delhi.

Mahadevan, T.M. 2002. Geology of Bihar and Jharkhand. Geological Society of India, Bangalore.

Singh, S. 1994. Physical Geography. Prayag Pustak Bhawan, Allahabad, India.
 




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