Ganga Basin

The Ganges basin is a part of the Ganges-Brahmaputra basin draining 1,086,000 square kilometres in Tibet, Nepal, India and Bangladesh. To the north, the Himalaya or lower parallel ranges beyond form the Ganges-Brahmaputra divide. On the west the Ganges Basin borders the Indus basin and then the Aravalli ridge. Southern limits are the Vindhyas and Chota Nagpur Plateau. On the east the Ganges merges with the Brahmaputra through a complex system of common distributaries into the Bay of Bengal. Its catchment lies in the states of Uttar Pradesh (294,364 km²), Madhya Pradesh (198,962 km²), Bihar (143,961 km²), Rajasthan (112,490 km²), West Bengal (71,485 km²), Haryana (34,341 km²), Himachal Pradesh (317 km²) and Delhi (1,484 km²), the whole of Bangladesh, Nepal and Bhutan. Several tributaries rise inside Tibet before flowing south through Nepal. The basin has a population of more than 500 million, making it the most populated river basin in the world.

The Regional Setting Situated in the Northern half of the Indian sub continent, the Ganga- Brahmaputra basin constitutes the second largest hydrologic region in the world. The total drainage area of the basin region is about 17 4 million hectare spreading over India, Bangladesh, Nepal and Bhutan in South Asia and Tibet region of China north of Himalayan divide. Nestled amidst the natural watersheds of Himalayas in the north (and beyond into Tibet), Vindhayas in the south, the Aravalis in the west, Patkai, Naga and Lushai hills in the east. The Ganga-Brahmaputra basin is a single, sprawling, integrated, transboundary drainage system with a common terminus. The basin consists of two major river systems of the Ganga and the Brahmaputra and one relatively smaller river system of the Barak, which becomes the Meghna in Bangladesh. About 63 percent of the basin is in India, 7 percent in Bangladesh, 8 percent in Nepal, and 2.5 per cent in Bhutan. Almost 10 percent of the world's humanity lives in this region, which contains only 1.2 percent of the landmass. The Physical Geography of the Ganga-Brahmaputra Basin The Ganga-Brahmaputra basin can broadly be divided into four geographical regions the Himalayan zone, the Great Plains, hilly areas of east and hill and plateau tracts of the south. The Himalayas The Himalayas, with some of the loftiest peaks of the world is one of the youngest mountain regions on earth and is still rising owing to the tectonic activity. The Himalayas form a highly rugged and continuousstretch (2400 km), between the gorges of Indus and Brahmaputra. Their width varies from 500 km in Kashmir to 200 km in Arunachal Pradesh. The entire Himalayan area as well as plain remained under the level of the sea till comparatively recent geological times. The region was occupied by a great geosyncline called the Tethys Sea as late as Mesozoic times. The sediments deposited in the Tethys Sea were folded to acquire the present form of the Himalayas and the associated mountain chains, in the very last phase of mountain building in the earth's history. This tertiary mountain building [or orogenic phase] is often referred to as Alpine after Alps, which were also uplifted roughly at the same time.l The modern Plate Tectonic theory explains the mountain building phases of the Himalayas. The Himalayan ranges were formed when the Indian plate was driven northwards and pushed beneath the Eurasian plate. With the advance of the Indian plate towards the north, the Tethys started contracting about 65-70 million years ago. About 30-60 million years ago, the two plates came closer and the Tethys sea crust began to fracture into thrust edges. About 20-30 million years ago, the Himalayan ranges started emerging. Intensive erosion from these mountain ranges resulted in the deposition of molasses in a basin towards south of Himalayan range. The Himalaya is still rising at the rate of 7.5 - 10 em per year.2 The Himalayas comprises of three almost parallel fold ranges interspersed with deep valleys and extensive plateaus (1) Siwaliks - The Siwaliks represent the outermost range of the Himalayas made up mostly of sediments brought by rivers from the main Himalayan ranges situated farther north. These hills have roughly a hog back appearance with relatively steeper slopes towards south. These hills form almost a continuos chain to the south of Himachal [Middle Himalaya] from Jammu to Arunachal Pradesh. Theirheight varies from 600 to 1500m. The Dangh Range, Dundwa Range and the Churia Ghati Hills of Nepal form parts of the Siwalik range. Most of the Himalayan Rivers, which have their sources in the Middle or Greater Himalaya or beyond in the Tibetan plateau, pass through the Siwalik range through wide and often steep gorge like valleys. The gorges of the Teesta and Raidak in West Bengal have joined and form a 90 km wide gap in the Siwalik range. Flat bottomed, cultivated valleys called "Duns" in the central part, covered with thick gravel and alluvium, separate the Siwalik range from the Middle Himalaya. The Dehradun Valley, 75 km long and 15-20 km wide, is a typical depression of this type. The range is covered with tropical wet deciduous forests in the east upto the Nepal; the vegetation becomes thinner farther west. Middle Himalaya Lying to the north of the Siwalik range, the Himachal or Middle Himalaya ranges form a most intricate and rugged mountain system. The range is 60-80 km wide and 1,000-4,500 in height, with several ---peak-s-rising-up"fo-5~000in.- In-the-Eastern Section few ranges can be marked clearly like Mussorie and the Nag Tibba ranges. The Mahabharat Lekh of Nepal is important Middle Himalaya range running through the country west to east. Its height varies from 1,500 to 2,000 m above mean seal level. The Middle Mountains form the great Central belt of Nepal and cover 30 per cent of the Kingdom. This zone, which has been long been under extensive agricultural use, is composed almost entirely of mosaic ridges and valleys with less than 5 per cent being flat land. The Great Himalaya The northern most Range of the Himalaya is a long continuous,inner most and loftiest range. It is the highest range in the world with anaverage height of 6,100 meters above sea level. One of its peaks, the Mount Everest (8,848 meters) situated at the northern border of Nepal, is the highest peak in the world. Its other notable peaks are Kanchanjunga (8,598 meters), Makalu (8,481 meters) Dhaula Giri (8, 172 meters) etc. 3 This range is forbidding and can be crossed only by a few passes which too are snow bound during winter months (generally higher than 4,570 meters above sea level), for example Nathu La and Jelep La in Sikkim. 4 Bhutan is a land locked country and almost entirely mountainous with flat lands limited to the broader river valleys. With altitudes ranging from 200 m to over 7,500 m., it displays enormous physiographic and ecological diversity. The complex geological history of the Himalaya and the tectonic stresses thrown up by dynamic geological process of seismicity, weathering, mass wastage and erosion affect and modify the environment of the basin. The central axis of these ranges consists of crystalline rocks, granites, gneisses and metamorphosed sediments ranging in Pre- Cambrian to as late as Miocene. To the south of this crystalline axis, occur mixed zones of sedimentary rocks and metamorphies, which are highly folded and faulted. The northern contact of these sedimentaries with the central axis is well marked tectonic feature called the Main Central Thrust (MCT), along which the crystallines are thought to have moved partially over the sedimentary zone. Another major tectonic feature called the Main Boundary Fault (MBF) south of the MCT and running almost along the entire Himalayan axis from Kashmir to Assam, makes the southern boundary of the sedimentaries and the northern margin of the Siwaliks. The geologically young Himalaya, with its residual stresses and highly compressed and tectonised rocks, is very susceptible to weathering and damage by seismic activity. The problem is further compounded by thesteep slopes (300 to 400) of these mountain systems, which results in heavy landslides and erosion. Human interference in form of various development activities and the quest for arable lands aggravate the geologically dynamic processes that lead to disasters and damage to these mountain's ecosystems. Any development envisaged in the basin has therefore, to take into account the geological hazards.s Role of Himalaya The Himalayas do not only shelter the Great Plains in south from the cold winds of central Asia in winter and check the moisture laden South West Monsoons causing rainfall in the plains in summer, but also they are mainly responsible for the Monsoon type climate prevailing in the subcontinent. The Himalayas acts as a natural frontier separating India, Bhutan and Nepal from China. The Himalayas constitutes the largest reservoir of snow and ice in the world outside the Polar Regions. Nearly fifteen thousand Himalayan glaciers as well as the large snow cover, estimated to be 1,400 cubic kilometers in volume, constitute a vast water storage system. The figure of glaciers in the Indian part of Himalaya according to an exercise taken Geological Survey of India safely be taken as 3,000. Nepal and Bhutan may similarly have 3, 000 glaciers. The Tibetan Himalaya also has equally large number of glaciers, many of which drain into the Brahmaputra, Sutlej and Indus. Around 3,500 glaciers probably drain into Ganga basin while 611 glaciers drain into the Teesta and Brahmaputra from southern slopes. It accumulates over the winter and draining into many rivers and tributaries through the summer dry season, which is matter of great significance to the economy of the plains below Drainage System of the Himalaya The Brahmaputra has its sources near Lake Mansarovar in Tibet. The Greater Himalaya Range is bout 1,500 in higher than the level of the source region of these rivers. It crosses the Great Himalaya through a very deep and narrow gorge before entering India.6 According to the geologists, the present course of the river was already in existence before the Himalayas folded. Subsequently, as the Himalayas rose, the river cut their beds deeper and deeper leading to the formation of the present gorges. The rivers thus form antecedent drainage through the Himalayas. Headwaters of many other rivers including the Bhagirathi, Alaknanda, Karnali, Gandak, Arun Kosi, Teesta and Manas have cut deep gorges in the Himadri on its southern slopes. The Great Plains The Great Plains stretch in an east west direction between the Himalayas in the north and peninsular plateaus in the south. The Great Plains is a depositional surface of great extent formed after the Himalayas. Their average depth is 1,300-1,400 meters. The crescent of alluvium, from the Ganga Brahmaputra delta in the east to the Indus delta in the west, probably represents the infilling of a fore deep warped down between the peninsular block and advancing Himalayas. Rivers emerging from the Himalayas deposit their load in fans along the foothills. These fans have merged together to form a 10-15 km wide piedmont plain of gravel and unassorted sediments called "Bhabhar", which forms the northern boundary of the Great Plains. 7 The Terai belt is marked by a reemergence of the streams on the surface from the Bhabhar belt. It is a zone of excessive dampness, with a thick growth offorest and variety of wildlife. Terai Zone occupies about 19 percent of the geographical area of Nepal. The Ganges plain in upper part has gradient of about 25 em per km., where river bluff's, meanders, ox- bow lakes, sandy stretches and nver channels break the monotony of the flat and featureless plain. It is drained by tributaries like Yamuna, Ghaghra, Sarda, Gomti etc. The Middle Ganga Plain is drained by Gandak and Kosi which flow sluggishly to form levees, bluffs, ox bow lakes, marshes, ravines etc. Almost all the rivers keep on shifting their course making this area prone to frequent floods. The Kosi River is notorious in this aspect. The Lower Ganga plain is formed by the sediments deposited by the Teesta, Jaldhaka, Torsa and old alluvium tract of Kosi- Mahananda and Sunkosh. The huge delta of Ganga form two third of this plain, where the Ganga divides itself into several channels (slope of the land is mere 2 em per km). The seaward face of the delta is studded with large number of estuaries, mudflats, mangrove swamps, sandbanks, and islands and fore lands. The impenetrable Sundari forest called Sunderban covers large part of the coastal delta. The Brahmaputra plain IS an aggradational plain built up by the depositional works of the Brahmaputra and its tributaries. Bangladesh is dominated by the combined delta of the Ganges, Brahmaputra and Meghna rivers. The country is laced with numerous distributaries, tidal creeks and spill channels. Along the South Western coast, the Sunderban is a heavily forested swamp area with numerous low islands.8 The Aravallis, the Vindhayas, Baghelkhand and Chotanagpur plateau and the eastern frontier hills which form the western, southern and eastern boundaries of the basin respectively are geologically old, ranging in the age almost from the oldest to the Tertiary.

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