Does the Dam strangulate the river?
I saw many misconceptions about dams are floating around different facebook communities (link1, link2). There are a lot of pictures showing dams as they are restricting flows of rivers and as if reducing the water supply of the channels. This is nothing but an anti-dam propaganda. Dams do have a lot of bad effects, most of them are ecological and environmental in nature. Besides, an unplanned dam can have a high running cost. But no way they should reduce the water flow downstream, if there are no major river diversions planned along with it.
In Indian subcontinent, most of the dams capture additional volume of water during rainy season and they release them during dry season so that they can achieve significant flood moderation. This is the way India achieves high water usage. India gets only 1750 cubic meters of water per year per capita. Ethiopia gets only a hundred cubic meters less. Although, Ethiopia runs into drought every couple of years and have constant food insecurity, India does not have similar problems. This is because Ethiopia can use only 2% of their water resource while India uses 34% (source : UNESCO). There is a bitter political story for Ethiopia which I don’t want to discuss right now.
Coming back to dams, the true picture of a dam can be revealed only if we observe the history. We need to see how it evolved after the dam was built. The pictures those show dams to be sucking water from the rivers are like ones displayed below.
These pictures can paint an image of a dam that strangulates a river and reduces the flow downstream. The viewer often gets the feel that the water volume before the dam was “natural” and the dam made it to reduce it to the flow we see coming out of the dam. Even I used to carry similar feelings a few years back when I visited a dam site in Karnataka. I saw a big wall separating the water reservoir from a thin flow of water coming out of it. I thought people upstream might have caused a lot of trouble to the downstream states by erecting it. Later I found out, it went as an win-win for both of those states – Mysore and the British India – as they both gained out of it. The engineer (Sir M. Visvesvaraya) who was the architect of it, was awarded both British and later Indian highest awards. Till date, India celebrates his birthday as Engineers’ day.
So what actually happens when a dam is built in a river? To find out the truth, one needs to see the picture of the dam before and after it is constructed. After a string of google searches, I was able to gather a few images.
As you see in all theee images above, the true river flow before the dam was built was that of what is coming out of the dam. The dam holds the water when it rains and the river flow goes beyond normal. It releases this excess water when the river is dryer. It acts as a buffer to moderate the natural extremeties observed in the river. In a lot of cases, hydroelectricity is also been produced from the same dam site. I hope the readers are able to understand this properly.
One good example of why dams cause trouble is also depicted in the first image of Glines Canyon Dam, which happened to be located at the same state I live in – Washington. It obstructed natural Salmon habitat. The wiki page says :
“Lacking passage for migrating salmon, its construction blocked access by anadromous salmonids to the upper 38 miles (48 km) of mainstem habitat and more than 30 miles (48 km) of tributary habitat. The Elwha River watershed once supported salmon runs of more than 400,000 adult returns on more than 70 miles (110 km) of river habitat. Today, less than 4,000 adult salmon return each year in only 4.9 miles (7.9 km) of river.”
This forced US Government to restore natural habitat of Salmons through “Elwha River Ecosystem and Fisheries Restoration Act of 1992”. The dam has been decommissioned though the Salmons are not back yet.
In contrast, the dams in Bangladesh (Kaptai) and India (Damodar) significantly reduced the number of Gangetic Dolphins, who depend on seasonal change in waterflow for migration. However, no steps were taken to decommission the dams associated. In third world, the scenario is totally different – hungry people would have anyway probably killed these Dolphins. There are no public movements at all to save this species – welcome to the reality of poverty in the third world.