Are Hydroelectric Dams as Green as We are Led to Believe?

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How Green is Hydroelectric Power?

Dams are man-made structures built across rivers usually to control the flow, regulate flooding and improve navigation. But, touted as an affordable, and greener natural source of renewable energy that produces minimal emissions of greenhouse gases, dams are now being built to produce hydroelectric power. As much as 70% of electricity supplied to the Pacific Northwest is provided by hydropower, with 40% of all US hydropower originating from the Snake and Columbia Rivers. But how environmentally friendly is this ‘green’ source of energy?

Environmental Effects of Dams

Dams disrupt both the flow and natural ecology of rivers. Plants and animals that inhabit the river system have adapted to the natural cycles of flood and drought, and when these natural cycles are disturbed, the natural cycles of aquatic fauna and flora are disrupted. Since dams restrict the flow of water downstream, aquatic species, such as salmon, whose reproduction cycles are timed according to the annual flood season, may be adversely affected. However, it is not only these disrupted natural cycles that impact fish and other aquatic species. When flood-gates on dams are opened, the river downstream can change from a trickle to a raging torrent in a matter of seconds. This has serious implications for the river downstream, as well as the species that inhabit the river – it can cause severe erosion of riverbanks, and result in rapid fluctuations in water levels and water temperatures, which can be fatal for fish and other aquatic species, as well as birds and other animals that nest or dwell in the vegetation on the banks of the river.

Downstream plant and animal communities depend on the river providing a constant supply of debris, such as fallen leaves, twigs, and branches, as well as the carcasses of dead animals. This debris is not only an essential source of nutrients for plants and food for animals downstream, it also provides microhabitats, providing a source of shelter and protection from predators. Dams block the flow of water, retaining debris behind the dam wall, which reduces the supply of nutrients and habitat that organisms downstream depend on for their survival.

Dams also prevent fish from migrating. Salmon 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. Fish ladders at dam walls may help fish to migrate upstream, however, most do not swim safely back downstream as they do not make it through the hydroelectric turbines alive. Others may be fatally injured when they swim over dam walls or suffer from barotrauma from the sudden change in pressure. This may negatively impact the food supply of local communities – both animals and human – 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, recreation and tourism.

Greenhouse Gas Emissions

Research has shown that large dams contribute a significant amount of greenhouse gases – both carbon dioxide and methane – into the atmosphere. As more large dams are built, trees on the valley floor that were absorbing and removing CO2 get swamped as the dam is filled. These trees eventually decompose on the dam floor, together with organic debris carried into the dam from upstream that sinks to the dam floor after getting trapped behind the dam wall. Methane gas (sometimes referred to as swamp gas) is formed when organic debris decomposes in an oxygen poor environment – the muddy, oxygen-free sediments on the dam floor provide ideal conditions for methane production. This methane is released at the surface of the dam, as water gushes over the dam wall or flows through turbines, and when water levels drop following dam drawdowns. As a greenhouse gas, methane is even more potent than carbon dioxide – 25 times more potent to be precise. Nearly a quarter of all anthropogenic methane emissions come from dams – this is rather a significant contribution.

Greening Hydroelectric Power

While hydro-electric dams do have a number of environmental disadvantages, hydroelectric power does offer a renewable source of energy, as long as the river continues to flow. Environmental organizations such as American Rivers and International Rivers have advocated the removal of obsolete dams and improving existing dams to minimize their impact on the river system. They also recommend that rather than constructing new dams on rivers, existing dams should be upgraded for hydropower generation where feasible. However, these upgrades should be carefully planned, and should take cognizance of the river’s ecology to limit negative environmental effects.

An excellent animation illustrating dam effects can be viewed at

Reduce the Pressure on Our Rivers

We can all do our bit to reduce the demand for electricity, and thus the pressure on the rivers that produce that electricity. Try to make your home as energy-efficient as possible so that less energy is needed from our local providers, or consider installing solar panels for a continuous supply of free green energy.