Dams are man-made structures built across rivers usually to control the flow, regulate flooding and improve navigation. But due to the effects of climate change, dams are now being built at a greater pace to produce hydroelectric power – toted as an affordable, and greener natural source of renewable energy that produces minimal emissions of greenhouse gases. Researchers have been searching for environmentally friendly alternative sources of energy to meet the world’s growing energy demands, and hydroelectric dams seem to be a viable option. Let’s take a look at how environmentally friendly these dams really are.

Environmental Effects of Dams

Dams disrupt the natural ecology of rivers. Plants and animal communities that inhabit the river adapt to the river’s patterns of flood and drought, and once that pattern is disturbed, it interrupts the natural cycles of aquatic species. As dams restrict the flow of water downstream, aquatic animals that coordinate their reproduction with the annual flood seasons may be largely affected. Furthermore, a dry river bed can be turned into a raging torrent in a matter of seconds; this not only causes erosion of the riverbank, but rapid fluctuations in temperature and water levels can kill fish, and other aquatic organisms, and even animals and birds that nest in the riparian zone.

Downstream river inhabitants depend on the river feeding a constant supply of debris, including leaves, branches, twigs and the organic remains of dead animals. Not only is this organic debris an important source of food and nutrients to downstream plant and animal communities, it also provides microhabitats for a range of species – offering shelter and refuge from predators, and providing a substrate for microorganisms and algae to grow. Dams hold back debris, and consequently reduce the flow of both nutrients and habitat for downstream organisms.

Dams also prevent fish migration. Salmon, for example, require a specific habitat to breed, and migrate upstream to reproduce. The barrier imposed by the dam prevents them from being able to do so, causing these populations to decline. While fish ladders may assist fish to move upstream, not many make their way back downstream through the hydroelectric turbines alive. This may negatively affect the food supply of local residents – both human and animals – that depend on these fish for their survival. It can also affect the livelihood of local people and businesses due to the loss of income opportunities from farming, fishing, tourism, and recreation.

A Solution to Climate Change – or NOT?

Researchers have concluded that large dams actually contribute a substantial amount of carbon dioxide and methane – gases responsible for climate change – to the atmosphere. As more dams and reservoirs are constructed, trees that are actually available to absorb carbon dioxide are swamped by water, where they eventually rot at the bottom of the dam. Furthermore, organic debris that is carried downstream accumulates behind the dam wall, and eventually sinks to the floor where it decomposes. This results in Methane gas (also known as swamp gas) that is formed when organic matter decomposes in the absence of oxygen. The deep, dark, muddy sediments at the bottom of a dam provide an anaerobic environment that is conducive to the production of methane, which is released through the air/water surface interface, and when water flows over dam walls, or gushes through turbines. Methane gas is 25 times more potent than carbon dioxide in terms of its greenhouse effect. Methane emitted from dams accounts for 23% of total anthropogenic methane gas emissions – rather a substantial contribution.

So, while dams provide healthy alternative to burning fossil fuels, there are some substantial environmental impacts to consider, most importantly the impact to the natural aquatic life and surrounding life that relies on these rivers.