Dr. Hrishikesh Baruah
Asst. Professor, Department of Geology, Arya Vidyapeeth College,
Guwahati: 781 016
Rivers have not only helped civilizations to sprout as well as inspire poets to enlighten the world, since geological times rivers have been utilized by nature to host several biological evolutionary events, act as habitat and sculpture the landscape. Nature also exploits the drainage network to dissipate energy that otherwise concentrate in the catchments. Dams happen to be a popular mechanism developed by mankind to extract fluvial energy for its own benefit. Potentially, dams also provide many benefits like reducing flood hazards, supply drinking water, support irrigation, provide recreational opportunities and improve certain aspects of the environment.
Adverse environmental and sociological impacts have also been identified during and after many dam constructions both on the upstream (reservoir) as well as downstream sides. Downstream impacts are however seen many hundreds of kilometres downstream, and well beyond the confines of the river channel. Whether such projects are ultimately beneficial (as projected) or detrimental to both the environment and surrounding human populations have been debated since the 1960s and probably long before that, (c.f., www.wikipedia.org). More recently, the Three Gorges Dam and other similar projects throughout Asia, Africa and Latin America have generated considerable environmental and political debate. By the way, let’s not forget the imbroglio at our own backyard.
Downstream impacts of dams are mostly unrecognized, misunderstood and underestimated during the planning stage, (Adams, 2000). Traditional assessments of dam’s stress on the investment to build it, the annual operating cost and compares the same with direct benefits projected from services like irrigation, power, and flood protection etc that a dam would provide. Such assessments fail to quantify environmental impacts, loss of natural resources and production opportunities, forced resettlement, and social dislocation, etc. Indeed, downstream impacts could be very somber.
Social impacts of dams in downstream environments consequences from multifarious interactions between environmental and economic impacts. Communities whose existence, age old customs and traditions revolve around rivers are influenced all along in a number of ways due to dams. The enormously complex aquatic and floodplain ecosystems as well as river wetlands provide avenues for agriculture, hunting, fishing, grazing, and gathering to the river margin communities. Changes in river flow regimes have very large potential effects on river and floodplain environments, and hence on the people depending on the resources of these ecosystems. Even the distribution of groundwater that river flow supports in space and time is affected. Floodplain agriculture is a high-risk tricky affair wherein planting and harvesting is synchronized with onset and recession of flood. Dams regulate flooding and thereby fertility, and in so doing make the farmers who were earlier skillful in this practice vulnerable. Dams can have significant effects on fishing communities for many hundreds of kilometers downstream. River bed degradation downstream of dams deteriorates spawning grounds for fish. The problem aggravates further as dams not only depreciate downstream water quality but also changes natural flooding pattern thereby deflating the much needed stimulus for fishes as well as other riverine biota to flourish.
Accelerated water flow downstream due to dams acquires the prospect to threaten far away shoreline ecosystems. Dams shrink sediment load reaching coastal plains and deltas and thereby abridges support for wildlife too. Nile delta is experiencing a host of problems due to the Aswan Dam. There is significant erosion of coastlines due to lack of sand, which was once brought by the Nile all along the eastern Mediterranean causing great harm to the brackish water fishing industry.
Carney (1998) identified five forms of capital that fortify the assets of the poor: natural capital (natural resources), financial capital, physical capital (infrastructure to process natural capital to usable product), human capital (skills, knowledge) and social capital. Downstream impacts disarray the tuned interrelationships between these capitals thereby impacting the societal fabric. Isn’t it surprising that despite years of research and quite a large number of experiences globally to be considered as case studies on downstream effects of dams on rivers, only a few general models for predicting how any particular river is likely to adjust following impoundment could be framed till date?
Policy makers, planners, scientists, agencies and intellectuals point fingers at each others when things doesn’t turn out as surmised on paper. Common mortals mostly play the role of mere spectators. Most of the impacts of river engineering are extremely complex and in many cases impossible to predict with certainty, and mankind is still in the process of understanding the intricate relationships between different elements of nature. Every river is unique in terms of its flow patterns, the landscapes it flows through and the species it supports. The relationship between a river and the landscape that it flows through is an affair of constant forceful adjustments maneuvered by climate and tectonics. Dams modify the pre-existing geologically controlled transport regimes and introduce new processes (disturbances) into the channel system. Basically, dams affect the geomorphic system by manipulating the ability of the river to transport sediment and the amount of sediment available for transport.
Downstream effects of dams on rivers can be scaled by the degree to which the dams change the predam flow and transport regimes, (c.f., Brandt, 2000). Typical downstream responses are channel bed degradation, textural changes like coarsening or fining of surface grain-size distributions, and lateral adjustments like expansion or contraction of channel width.
Understanding of the regional geologic setup helps to visualise the trajectory of changes that are likely to hit the terrain planned to be ornamented with a dam. Geologic setting has direct control over the watershed, channel properties and sediment load through a hierarchical set of inter-linked processes. At the scale of a drainage basin, the geology, including both physical properties of the underlying rocks and their structural features and tectonic deformation, interacts with climate to produce topography, including relief and the drainage network pattern, (c.f., Starkel, 2003). As such, detailed studies of interactions between climate and tectonics is always helpful while planning something as big as damming the Himalayan rivers. While tectonics is a major player in regulating internal dynamism of the earth, climate commands similar clout on the exterior front of the planet.
The tectonically charged setup of North East India is known to all. Climate here plays a big role too. The palaeoclimatic scenario of the Indian sub-continent during the last 15K years suggests that the dynamism of the Himalayan Rivers have been largely influenced by the SW monsoon, (c.f., Tripathi et al., 2004).
Such inferences could be drafted vide studies on pollens, sediment characteristics, palaeosalinity records, geomorphic and structural entities to name a few. Yet, in the NE Himalayas that caters to the water resources capital of the nation the relationship between climate, erosion, deposition and tectonic activities is yet to be convincingly understood. Records suggest that erosion, river hydraulic energy and sediment load is always high in the Himalayan Rivers in consonance with a vibrant and energetic monsoon over the years. Highly stimulated monsoon invigorates denudation and river dynamism which further can contribute to tectonic movements. Evidently, in the whole Himalayan region the Brahmaputra basin experiences relatively higher rates of uplift due to intense erosion and subsequent isostatic (read, height) adjustment, (Singh, 2006). Srivastava et al. (2009) in their detailed studies in and around the Pasighat area of Arunachal Pradesh have come across many evidences proving high interaction of climate and tectonics. Somewhat inherent disciplinary bias in dam planning process influences downstream impacts which are seen as a secondary problem, to be addressed once the technological viability of the project is known. However, the initial investment of technical evaluation is such that it becomes hard to stop a project when the stage for incorporation of full social costs comes.
W. Adams from the University of Cambridge in his research paper ‘Downstream Impacts of Dams’ submitted to the World Commission on Dams in 2000 opined that analysis of the impacts of dam’s impacts should be holistic in spatial, social and economic senses. A mechanism to monitor and periodically re-examine the impacts of dam development in downstream communities should be an integral element of the planning process. The USGS (United States Geological Survey) for example, does the monitoring and provide dam managers and resource planners with the kind of in-depth information they need to reduce some of the more harmful downstream effects of dams on river environments. All people for whom the natural flow of the river and its associated resources are natural capital to survive and sustain in life must be adequately benefited. Women, children and the elderly, tend to be impacted by dams more and as such, project planning should allow for the participation of people affected by project development in downstream areas.
