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The Teesta River (Pron:ti:ˈstə) or Tista (Nepali: टिष्टा, Hindi टीस्ता Bengali তিস্তা) is said to be the lifeline of the Indian state of Sikkim, flowing for almost the entire length of the state and carving out verdant Himalayan temperate and tropical river valleys. The river then forms the border between Sikkim and West Bengal before joining the Brahmaputra as a tributary in Bangladesh. The total length of the river is 309 km (192 mi) It drains an area of 12,540 km². Before a large part of this was situated in Nepal. But after the Sugauli Treaty it was acceded to British India.
The Teesta River originates from the Pahunri (or Teesta Kangse) glacier above 7,068 m (23,189 ft), and flows southward through gorges and rapids in the Sikkim Himalaya. It is fed by rivulets arising in the Thangu, Yumthang and Donkia-La ranges. The river then flows past the town of Rangpo where the Rangpo River joins, and where it forms the border between Sikkim and West Bengal up to Teesta Bazaar. Just before the Teesta Bridge, where the roads from Kalimpong and Darjeeling join, the river is met by its main tributary, the Rangeet River. At this point, it changes course southwards flowing into West Bengal. The river hits the plains at Sevoke, 22 km (14 mi) north of Siliguri, where it is spanned by the Coronation Bridge linking the northeast states to the rest of India. The river then courses its way to Jalpaiguri and then to Rangpur District of Bangladesh, before finally merging with the Brahmaputra River at Fulchori.
Through its course, the Teesta river has carved out ravines and gorges in Sikkim meandering through the hills with the hill station of Kalimpong lying just off the river. Variegated vegetation can be seen along this route. At lower elevations, tropical deciduous trees and shrubs cover the surrounding hills; alpine vegetation is seen at the upper altitudes. The river is flanked by white sand which is used by the construction industry in the region. Large boulders in and around the waters make it ideal for rafting enthusiasts.
Between Rangpo town and the railway bridge (popularly called Lohapul or iron bridge) on it as it enters the plains at Sevoke, the Teesta flows with a very strong current, ideal for white river rafting. Towns like Teesta Bazaar and Melli have facilities for group rafting. Though the river looks innocuous, the underlying current is very strong. In 1915, G.P. Robertson, the then Municipal Engineer of Darjeeling, drowned after losing control of the boat in the turbulence while surveying the river. The boat struck a partially hidden boulder and was sucked in by a whirlpool, leaving no trace of the occupants.
Great changes have taken place in the course of some of the rivers in Bengal and the adjoining areas, during the period since 1500 CE. Although positive evidence is lacking, similar changes can be assumed in the remoter past. The Teesta River is one of the rivers that has changed over the years.
The Teesta earlier ran due south from Jalpaiguri in three channels, namely, the Karatoya to the east, the Punarbhaba in the west and the Atrai in the centre. The three channels possibly gave the name to the river as Trisrota "possessed of three streams" which has been shortened and corrupted to Teesta. Of these three, the Punarbhaba joined the Mahananda. The Atrai passing through a vast marshy area known as Chalan Beel joined the Karatoya and the united stream joined the Padma (Ganges) near Jafarganj. In the destructive floods of 1787, the Teesta river forsook its old channel and rushing south-east it joined the Brahmaputra. James Rennell made a survey between 1764 and 1777 and his maps are one of the earliest authentic maps of Bengal in existence. In these maps Teesta is shown as flowing through North Bengal in several branches — Punarbhaba, Atrai, Karatoya, etc. All these streams combined lower down with the Mahananda, now the westernmost river in North Bengal, and taking the name of Hoorsagar finally discharged into the Ganges at Jafarganj, near modern Goalundo. The Hoorsagar river is still in existence, being the combined outfall of the Baral, a spill channel of the Ganges, the Atrai, the Jamuna or Jamuneswari (not the main Jamuna through which the Brahmaputra now flows), and the Karatoya, but instead of falling into the Ganges, it falls into the main Jamuna, a few miles above its confluence with the Padma at Goalundo.
India has proposed a series of dams within the Teesta river system that should produce some 50,000 MW of electricity within the next 10 years. With some of the largest sediment loads, the creation of a reservoir will lead to an increased pressure on an active fault area. There are concerns that the building of these dams may lead to river-induced seismicity. Despite such worries the construction of the dams had started. Links are suspected between the dam construction and the deadly 2011 earthquake in Sikkim.
The Teesta river has preserved good imprints of climatic and tectonics along its valleys and catchments. The interrelationship between climate, erosion, deposition and tectonic activities is not properly understood to date. However, it appears that major alluviation and incision events could be ascribed to the factors associated with climatic processes such as strengthening or weakening of monsoonal precipitation and related fluvial discharge. Tectonic activity affects sediment fluxes and is responsible for the insetting of younger terraces/fanlobes into the older terraces/fanlobes. During seismic events, landslide activity along the slopes of river valleys influences sediment delivery into the valleys, causing the effects of tectonics to be intricately coupled with that of climate
The terraces and floodplains, valley-side slopes and landslide slopes, alluvial cones of different generations, kettle-shaped depressions, sickle-shaped ranges, leveled plains, undulating plains and deeply dissected valleys, and glacial and periglacial deposits are some of the geomorphological features observed in the Teesta river basin in Sikkim (Mukhopadhayay, 1982). Three prominent knick points have been observed along the Teesta river profiles which correspond to the zones of tectonic discontinuities, the important ones being the MCT and MBT (e.g. Seeber and Gornitz, 1983). Results of recent studies indicate that the southern part of the frontal wedge near the foothill zone is tectonically active along with the formation of NKT, SKT and MFT structures within the sub-Himalaya in the Teesta basin.