Rainfall – its nature and pattern – is probably the most important environmental factor regulating the biodiversity of a region. Unequal distribution and seasonal variation of rainfall is characteristic of India. About 80% of the total annual average precipitation of 4,000 cu. km. occurs during the three monsoon months, starting in June. In terms of total annual precipitation received per unit land surface, India is much above the global average. However, in terms of per capita water availability, the scenario is a matter of great concern. The annual per capita availability has steadily dwindled from 6,008 cu. m. in 1947 to 2,266 cu. m. in 1997 (TERI, 1998). At present rates of economic and population growth, by the middle of the current century, total water demand will almost equal India's total utilisable water resources.
The snow-fed streams and rivers of the northeast Himalayan region are formidable water resources that harbour and give rise to rich biodiversity. A prolonged rainy season (late May to October) in the region, especially in Arunachal Pradesh, causes regular floods in the neighbouring state of Assam. Policymakers have found it hard to ignore the availability of great volumes of monsoon run-off in the Himalayan rivers and this has traditionally encouraged diversion, storage and transfer of flows through the construction of dams in the hope that this will not only 'tame' fast-flowing rivers but also generate electricity to cater to regional and national needs.
Of the total estimated hydro potential of 84,044 MW in the country, the fast-flowing Himalayan rivers are estimated to have the potential to generate about 80% (65,000 MW). Arunachal Pradesh tops the list with a series of hydel dams proposed for the rivers of Arunachal Pradesh – Siang, Subansiri, Ranganadi, etc., expected to generate 26,747 MW power, apart from controlling floods. The construction of dams will reduce the water level of these rivers and also the main river (the Brahmaputra) by 0.3 to 0.5 m. throughout its course. This, however, will exert more pressure on mega-fauna, which already face water scarcity in the dry months.
The Brahmaputra, being a large river, covers different climatic zones, landscapes and bio-geographic regions. Geomorphologically, the Brahmaputra basin is very unstable as it is located in a high seismic zone and is constituted by alluvial soil. Further, the river carries one of the highest sediment loads in the world, about 332 million metric tonnes annually throughout its course. Recent satellite photographs reveal that the Brahmaputra is continuously shifting southward and in some places, may be migrating at rates as high as 800 m./yr. The rate of aggradation is as high as 16.8 cm./yr. in the head reaches upstream from Dibrugarh (Varma and Rao, 1996). The shifting of the river is distinctly evident in the districts of Dibrugarh, Morigaon and Sonitpur, where the river has already shifted two to eight kilometres in the last ten years. Lateral migration of the channel is always associated with large-scale bank erosion, aggradations and widening of the river channel. Most of the erosion occurs along the southern bank at Rohmoria, Disangmukh, Nimatighat, Morigaon, Palasbari and in many places of Dhubri district. The river becomes shallow and ox-bow lakes are often formed due to changes in the river's course. The two most important factors for sedimentation or siltation in the river are:
- High sediment supply in the catchment areas after the 1950 and subsequent earthquakes, which caused landslides and uprooted trees.
- Heavy rainfall resulting in frequent floods loaded with high sediment concentration, which has given rise to a highly 'braided' river. In the catchment area, landslides are common during the monsoon months and tonnes of debris silt up the riverbed downstream. Although no hard data is available, the mean water velocity of the Brahmaputra appears to have decreased, resulting in silt deposition in the riverbed at a faster rate. The banks of the river are almost perpendicular and during floods, the loose texture of the soil, coupled with the strong current causes the erosion of the bank (Biswas et al, 2000). In recent years, large-scale logging has worsened the situation. Because of the wanton destruction of forests in the catchment areas and the continuous deposition of silt in the already raised riverbed, the river frequently floods during the monsoon. Erosion of the riverbanks and the char (riverine island) areas of the main river have resulted in the mouths of tributaries being dammed and feeder channels leading to floodplain lakes also being obstructed due to excessive sedimentation and a frequent change in the river's course. Consequently, the water passage to the floodplain lakes or beels is disrupted. These ecotonal zones play an important role in the dynamics of the Brahmaputra ecosystem, as these are natural feeding and breeding grounds for a number of fish species and other aquatic fauna.
Frequent changes in the river's course coupled with heavy siltation, especially in the upper reaches, have a great bearing on the river's faunal composition. As the river continuously widens with each passing year, the average depth simultaneously decreases and the water is distributed among numerous channels. As a result, water for large fish species such as the river dolphin becomes inadequate during the dry season. Shallow channels sometimes totally dry up in summer. Dolphins and turtles, abundant a couple of decades ago, are now seen only occasionally during the monsoon in select spots. In the event of the construction of dams upstream, the situation will worsen, as there will be even less water downstream, particularly in the dry season.
In the 1950s, large dams were considered the 'temples' of modern India. While Indian hydrologists still consider large dams to be the solution to water management, there are also growing protests against dams on account of their environmental impacts. As seen across the world, the channelisation and regulation of rivers, resulting in the reduction of floodplain areas and wetland drainage, causes serious modifications in the flow patterns of water, nutrients, sediments and pollutants. These catchment-wise changes have had severe impacts not only on river ecosystems but also on biodiversity at the landscape scale. A similar situation has been experienced in the Brahmaputra basin. Over the last three decades, approximately 4,000 km. of dykes (embankments) have been constructed along the Brahmaputra and some of its major tributaries as a flood control measure. It is now widely believed that these embankments do more harm than good. They are responsible for the shrinkage of feeding and spawning grounds of many prized fish species. The obstruction they cause has resulted in the disappearance of many of the state's spawn collection centres. A sharp decline in the catch of Indian major carps is a pointer to the loss of spawning grounds in the Brahmaputra system.
As a large number of hydroelectric projects have been planned upstream of the Brahmaputra, Siang, Subansiri and other Himalayan rivers, it is necessary to thoroughly study the possible ecological consequences. The impoundment and heavy accumulation of water will definitely cause the loss of many hill stream species such as garza, glyptothorax, hara, conta, etc. as they will be deprived of their usual fast-flowing habitat. Migratory fish species, both anadromous and catadromous, will find it difficult to reach their spawning grounds. Giant catfish such as Bagarius yarelli, Pangasius pangasius, Silonia silondia, Aorichthys spp. and Wallago attu migrate upstream when the river starts to swell after the pre-monsoon rains (April-May). In the subsequent floods (June-July), the carps move upstream and enter the tributaries, distributaries and floodplain lakes (beels) to breed. It has been recorded that species composition in the flood plain lakes largely depends on the intensity of floods. It is expected that the absence of 'high intensity' floods due to the construction of upstream dams will definitely affect the 'auto-stocking' of the lakes. Thus, the lakes might not be adequately stocked by riverine species. The profuse pre-monsoon growth of aquatic weeds will also not be 'flushed out' due to the inadequate flooding of the beels. The impoundment will also bring about drastic changes in the nutrient composition of the bottom soil influencing the diversity of benthos as well as other aquatic biota.
In the past, it was difficult to record the impacts made by engineers on the biodiversity of rivers. But the scenario is changing due to accelerated knowledge from the biological point of view that biota possess the capacity to regulate water flow through the landscape. Biological investigation of tropical and sub-tropical rivers are essential not only for administrative planning, surveillance and legislation, but also for a better understanding of the relationships between land-use change and water quality of the river in question. The evaluation of biodiversity in freshwater isotopes should be, in principle, based on precise identification of the biota in question. An indispensable precondition is precise taxonomic identification of the various groups of flora and fauna and their relative abundance in different seasons. A correct assessment of the population density of the different groups of biota is immensely helpful in establishing the relationship among the biota, especially food webs, which has become a central focus of interest in both limnology and general ecology. In addition, they are being appreciated as critical tools in freshwater biodiversity conservation and management. Therefore, there is an urgent need to effectively execute the Environment Impact Assessment as per the notification of 1994. The evaluation criteria should include impacts of the dam on the productivity of the surrounding soil, on genetic resources and loss of or damage to habitat both up and downstream of the river.
A methodology for the comprehensive assessment of dam projects with regard to impacts on ecosystems and people needs to be developed in consultation with relevant experts. A feasibility study must be properly executed taking into account the ecological as well as economic and social impacts on a regional level. Striking a balance between the need to maintain the ecological equilibrium of the river and meeting genuine needs for power, irrigation, etc. would result in a proper balance of economic development and biodiversity conservation.
Dr Sanchita Boruah is a Senior Lecturer of Zoology at D.H.S.K. College, Dibrugarh University and has been working on the eco-hydrology of the upper Brahmaputra river in Assam. Dr SP Biswas is the Principal Coordinator for Ecology and Fisheries of the Brahmaputra river and a Reader in the Department of Life Sciences, Dibrugarh University.
- Biswas, S.P., Baruah, D. and Hazarika, A. (2000) An experimental study of soil conservation using herbaceous plants in Majuli Island, Assam, India. The Environmentalist 20, pp. 19-27.
- TERI. 1998. Looking back to think ahead: GREEN India 2047. Pachauri, R.K. and Sridharan, P.V. (eds.). Tata Energy Research Institute, New Delhi.
- Varma, C.V.J. and Rao, A.R.G. (eds.), 1996. Aggradation in the Brahmaputra River in Assam, Central Board of Irrigation and Power, New Delhi, Publication No.252, pp. 1-9.