water rushing through a hydro-electric dam

photo credit Sky Light Pictures

TFor decades dams have been celebrated by some as an inherently clean form of energy. In British Columbia the Christy Clark government recently dubbed a highly controversial $9-billion mega dam on the Peace River as “the site C Clean Energy Project.”

Government propaganda adds that the dam “will provide British Columbia with the most affordable, reliable clean power for over 100 years.” But the science shows that damming water and flooding forest soils does not deserve the adjective clean. In fact, hydroelectric dams serve as a powerful reminder that every form of human-mined energy comes with significant ecological cost.

The destructive impact of some 300 hydro dams in Canada’s boreal forest is well known, but little acknowledged. Dams prevent the movement of fish, change flows in freshwater, release mercury, alter nutrient flows to the ocean, dry up deltas and flood large areas of forest and peat lands.

Also, building dams comes with a significant carbon footprint. It comes from one of two key sources: their energy intensive construction or the flooding and drawdown of water on carbon-rich lands.

Replumbing landscapes to create hydropower requires lots of heavy machinery and tonnes of concrete. The energy intensity of a medium-sized dam can produce the same amount of emissions as 46,000 vehicles a year.Cement, the second most consumed product on Earth, accounts for a whopping five percent of global CO2 emissions. But dams also make climate change worse by turning carbon sinks into methane bombs.

The mechanism is complex and has only recently been detailed. When engineers fill a dam, the flooding releases carbon that was tied up in trees and other plants as they rot. But that’s just the beginning.

The world's large dams release 104 million metric tonnes of methane annually. 

After vegetation settles on the oxygen-deprived reservoir bottom, it decomposes and pulses dissolved methane into the water column. Once that methane-rich water passes through a turbine, the methane is released into the atmosphere. The methane also spits into the atmosphere at spillways, and downstream of the dam.

According to Brazil’s National Institute for Space Research, the world’s large dams release 104 million metric tonnes of methane annually from reservoir surfaces, turbines and spillways. As a consequence the world’s 50,000 dams may be responsible for 23 percent of all methane emissions from human economies.

The Site C “clean energy project” is a perfect example. It would flood approximately 5340 hectares of prime agricultural land and generate emissions equivalent to 147,000 tonnes of CO2 per year or 36,000 vehicles.

Like BC, the Brazilian government has defended multiple dams constructed in the Amazon basin as sources of “clean energy” too. Yet due to the rotting of plant matter at the bottom of the reservoir, for example the Curuá-Una dam in Pará, Brazil, has produced three-and-a-half times more emissions than an equivalent generating plant running on oil.

Another Amazonian dam, Tucurui emits more climate changing gases than San Paulo, one of the most crowded cities in the world.

The methane problem does not just apply to tropical dams. In the United States, researchers in Washington state recently discovered that methane emissions from one dam site jumped 20-fold when the water was drawn down exposing rotting vegetation to the atmosphere.

In Switzerland even small run-of-the-river hydro projects can emit methane in significant quantities after nearly 100 years of operation. German researchers have also found that small dams only 15 metres deep can collect sediment and create methane hot spots that “can potentially increase global freshwater emissions by up to seven percent.”

So dams are not inherently climate friendly and the term “clean energy” is but another fiction designed to stop us from thinking critically about governments wedded to megaprojects.

Andrew Nikiforuk, a Calgary-based journalist, is the author of the national bestseller Tar Sands: Dirty Oil and the Future of a Continent. His latest book, The Energy of Slaves, looks at how human slavery has shaped our attitudes and values about energy. For more on Andrew visit his website at andrewnikiforuk.com


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