Evidences show that the natural flow of all rivers is inherently variable, and that this variability is critical to ecosystem functioning and native bio-diversity. The flow of the rivers sometimes changes due to geophysical changes.
But when people change the courses or discharges, the rivers gradually lose their existence. Rivers with highly altered and regulated flows lose their ability to support natural processes. In other words, dammed rivers are dead rivers.
In this write-up I would try to prove the above proposition through presenting example of gradual death of the mighty river Teesta.
Once the Teesta that used to hold water throughout the year now dries up just after the monsoon. Numerous chars and shoals have been emerged on the riverbed. The discharge capacity of Teesta has drastically been reduced due to withdrawal of water and the discharge of heavy silts from the upper catchments. A series of dams and barrages erected over the vibrant river are virtually causing its death. The shrinkage of the river has been causing heavy erosion almost throughout the year displacing and making destitute hundreds of people every year.
It seems certain that the dynamic equilibrium of the river will be impaired with the construction of a series of dams and the sediment load will be trapped within the reservoirs, reducing their capacity. This, in turn, could compel dam managers to release water during heavy rainfall, causing sudden flash floods downstream.
Origin of Teesta
The river Teesta originates in the mysterious Cho Lhamu Lake at an elevation of 5,330m (17,500 feet) above sea level in the mighty Himalayas. This lake lies to the north of the Donkia Pass near Shetschen, where the summit of the pass is about eight kilometres north-east of Darjeeling.
The Teesta is then fed by rivulets, which arise in Thangu, Yumthang and Donkia-La ranges and flows past the town of Rangpo where it forms the border between Sikkim and West Bengal up till Teesta Bazaar. At Teesta Suspension Bridge, which joins Kalimpong with Darjeeling, the river is met by its main tributary, the Rangeet river. At this point, it changes course southwards flowing entirely into West Bengal. The river hits the plains at Sevoke, where it is spanned by the Coronation Bridge which links the Northeastern states to the rest of India. The river then courses its way to Jalpaiguri and then to Lalmonirhat district of Bangladesh, before finally merging with the mighty Brahmaputra at Fulchhori in Gaibandha.
The Teesta is a rain and snow-fed river. The permanently snow-covered area of the basin is about 158.40 sq. km. The upper catchment receives a total annual rainfall of 1,328 mm. while the middle of the basin receives 2,619 mm. It has been recorded that about 77-84 per cent of the annual rainfall is received between June and September.
Dams on the Teesta
A number of dams and barrages have been built on the river Teesta on its 414 km journey to the sea from its source in Sikkim to the coast of Bangladesh. These include: Teesta Barrage in Bangladesh, Teesta Barrage Project at Gojoldoba in West Bengal, two hydro-electricity dams in Sikkim — one at Kulekhani and other at the upstream. The Indian government is also planning to construct two more hydro-electricity dams over the Teesta.
Teesta Barrage: The Teesta Barrage has been built at Doani in Lalmonirhat with an ambitious objective to bring 750,000ha of land under irrigation command area with net irrigation area of 540,000 hectares to augment agri production.
It is spread over 12 upazilas — Nilphamari, Dimla, Jaldhaka, Kishoreganj, Saidpur, Rangpur, Taraganj, Badarganj, Gangachara, Parbatipur, Chirirbandar and Khanshama. The project included construction of a barrage, flood embankment, flood bypass, silt trap, main canal and part of canal system with improvement of existing drainage canal.
Although the implementation of the project started in 1960, the actual construction of the barrage was taken up in 1979 and that of canal system in 1984-85. The first phase of the barrage has been completed in June, 1998 and the cost incurred is Tk. 9695.29 million.
Teesta Barrage Project (TBP) at Gajoldoba in West Bengal: The TBP is an overtly ambitious multipurpose project. It plans to irrigate 9.22 lakh ha of land in six districts of Indian north Bengal without any storage system. Three pick-up barrages are to divert river water towards agricultural land. The system may be successful for kharif cultivation when the soil is naturally wet and rivers are full. Since the rivers of north Bengal are much reduced in the lean months, it would be impossible to ensure water to dry variety paddy over 90,000 ha. The cumulative irrigation potential achieved by the project till June 2001 from its inception in 1976 was 12,6110 ha., which is less than 14 per cent of the ultimate target.
The TBP is excessively optimistic in its projections, especially considering that it has no reservoir and depends exclusively on diversion barrages with no storage capacity.
The Teesta was untamed in its upper catchment when the TBP was formulated. The series of proposed dams in the upper reaches will reduce the available discharge for irrigation as each hydro power project is expected to consume at least five per cent of the running water in the river. The lack of coordination between National Hydro Power Corporation (NHPC) and TBP has further complicated the situation.
The reservoir that was planned to be constructed during the second phase of the Teesta irrigation project cannot be now undertaken since the NHPC has already started working towards the implementation of the ‘low dam’ just 400m. upstream of the Coronation bridge. So the plan to generate an additional 600 MW power under the TBP will probably never take off.
The Teesta has been drying up at different points during the dry season threatening the boro cultivation in six northern districts. The once mighty Teesta is now bereft of water following construction of a barrage upstream at Gojoldoba point in Jalpaiguri of the Indian state of West Bengal.
The farmers in Nilphamari, Lalmonirhat, Gaibandha, Rangpur, Dinajpur and Bogra are worried over the bleak prospect of getting required quantum of water from the Teesta for the irrigation of boro fields. The construction of the barrage on this river across the border to divert its flow of water has badly affected the efficacy of the Teesta Barrage Project.
According to Water Development Board sources, Bangladesh got only about two per cent of the required quantum of water from across the border last year. The release of such low quantum water was affecting navigation, irrigation, fishery and ecology of our lower riparian country, the sources added.
On the other hand, they said, there should be 10,000 cusecs of water to bring an estimated 111,000 hectares under the Rabi crop programme but only 1,000 to 1,200 cusecs are now available in the upstream of the Teesta Barrage. The Indian authorities are reportedly withdrawing the total water from the rivers Teesta and Mohananda through their Gajoldoba and Mohanada Barrages in the upstream.
Pattern of discharge
It can be seen from the chart below that the average lowest discharge of Teesta was above 4,000 cubic metre/sec before construction of the two barrages — one at Doani in Bangladesh and other at Gajoldoba in West Bengal. But after construction of two barrages the lowest discharge has drastically reduced to 529 cum/sec in 2000 and just after five years in 2005 it came down to just 8 cum/sec. I think, there requires no further explanation what is going to happen to the fate of the Teesta in the near future.
On the other hand, in the Indian part, the mean annual discharge of the Teesta at Anderson bridge was about 580 cum/sec a decade back and it declines to 90 cum/sec in the lean months. The peak discharge may be as much as 4,000-5,000 cum/sec. It was estimated that the peak discharge of the river at Jalpaiguri during the devastating flood of 1968 was 19,800 cum/sec. The sediment load in the river increases with high monsoon discharge. It was observed that 72 per cent of the suspended load is transported between July and August when the bulk of discharge flows through the river.
The dams and barrages already constructed in the river Teesta have caused a negative impact on free flow of its water. Due to the obstruction on its water flow, the Teesta was heavily silted up and changed its courses at many places, especially in the lower catchment, and erodes its both banks engulfing thousands of hectares of land every year. Moreover, the ambitious objective of both the Bangladesh and Indian authorities of irrigating thousands of hectares of land to increase agricultural production is also gradually dwindling with scarcity of water during the lean period in the river. It was feared that the Teesta barrages both in Bangladesh and Indian may lose their efficacy within a decade. And by that time, the mighty river will not only become dysfunctional but also die in terms of water flow and replenishing its surroundings.
The Teesta is going to embrace the fate of the Aral Sea project in Russia and Irtsh-Karaganda Canal in Kazakhtan which have been proved to be ecological disasters of water management. The mighty river, flowing and replenishing its surroundings for thousands of years, is going to be almost vanished within a decade due to human interference with its natural course to accomplish their greed. And it can surely be said that the consequences of altering the nature would not ultimately bring any good for the people. As Fredrick Engels said “Let us not flatter ourselves overmuch on account of our human victories over nature. For each such victory, nature takes its revenge on us. Each victory, it is true, in the first place brings about the results we expected, but in the second and third places it has quite different, unforeseen effects which too often cancel the first.”
Nazrul Islam is a freelance journalist and environmentalist.
Source: The Daily Star, 03 November 2006