Generally speaking dams tend to impact Rivers in many ways. When the Three Gorges Dam was built, around 1.5 million people lost their homes and livelihoods. Dams come in variety of types and each type is designed for specific conditions and technical requirements. The Columbia River and its tributaries hold the record for the most dams at 60. This naturally leads to an intriguing question, how many dams can be built on a river anyway? 100? 1000? Is there any theoretical limit? The answer is not clear as you might think. Any limit on the number of dams that can be built on a river will depend on the type and purpose of dam. Dams can be built for a variety of reasons such as hydroelectricity production, water storage, flood control, navigation and recreation. But actually building a dam is only a part of story. They can’t just be built randomly. For example you can build a dam on a flat floodplain unless you want it to be practically useless. When engineers choose sites for dams, they tend to keep an eye on following criteria:
The topography and inflow of catchment area.
Morphology of valleys.
The underlying geology and geotechnical conditions of potential site.
Local climate and flood regime.
Any existing historical or ecological area that may be drowned by dam’s reservoir.
In an ideal world the site chosen will have enough inflow of water to maintain reservoirs all year around. It will also be located in a narrow site where a river valley widens upstream of the dam. The underlying geology will be impermeable, strong and thick and not dip towards the downstream of the dam. There should be limited or no jointing and faulting in order to prevent water leaking from the dam’s reservoir over time. It is possible to grout foundation geology to fill any holes but this just adds to the construction cost. A proposed dam will also not destroy any nature reserves, towns or places of historical, cultural or ecological importance. These issues will also determine the type of dam to be built. Dams come in variety of types and each type is designed for specific conditions and technical equipments. For example, a hydroelectric dam typically requires 39-49 feet (15-20 m) drop from the reservoir to water level of the point of discharge. Technical requirements like this will restrict the location and the number of this type of dam that can be built on a river. Another major restriction on the number of dams is money. Dams are significant constructions that cost a lot of money, materials and work hours to design and erect. It is almost impossible to generalize the amount needed to build a dam as it can vary hugely depending on location, type and size of the dam. According to some sources, a modest 25kW hydroelectric dam would cost around $230,000. A large 500kW dam would be somewhere in the region of 2 million dollars. Dams are also “costly” to environment since they can seriously affect the health of river. Generally speaking dams tend to impact rivers in many ways. The first is the fact that dams block river flow. This prevents fish migration for spawning, accessing feeding grounds and escaping predators.
When the Three Gorge Dam was built, around 1.5 million people lost their homes and livelihoods. 1500 cities, towns and villages were destroyed and countless rare architectural and archaeological sites and areas of natural beauty were lost.
Fish tend to form a key part of river ecosystem and any impact on their population can have a much wider impact on other animals. Dams also slow down rivers. Many aquatic organisms depend on steady water flows to survive. Any stagnation in water movement from reduced flow rates can disorientate organisms and destroy the natural growth and reproduction cycles of both plants and animals. The slowing down of water flow can also impact the water quality and create water temperature fluctuation that local organisms are not adapted to cope with. Oxygen levels can also drop leading to various ecological problems like algal blooms. There can be a heavy toll on humans too. When the Three Gorge Dam was built, around 1.5 million people lost their homes and livelihoods. 1500 cities, towns and villages were destroyed and countless rare architectural and archaeological sites and areas of natural beauty were lost. Dams on the Mekong and the Nile turned the areas downstream into deserts, destroying people’s livelihood along with environment. All of these issues affect the type, size and location of dams on a river. So, how many dams can be built on a river? It all depends.
(An author is pursuing Ph. D from Centre for Earth Sciences at the Indian Institute of Science, Bangalore. Views are author’s own)
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