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Canada's hydropower producers cheered in June when the leaders of Canada, the United States and Mexico committed their countries to raising the portion of continental electricity generated by clean-energy sources to 50 per cent by 2025, from the current 37-per-cent level.

It was a rare slice of good news for Canada's provincially owned hydro behemoths, whose risky bets on massive new power projects threaten to leave them (and taxpayers) saddled with billions of dollars in stranded debt amid plunging market prices for electricity. Their best hope for minimizing losses on Newfoundland's Muskrat Falls project, British Columbia's Site C dam and Hydro-Québec's Romaine River development lies in winning U.S. contracts that pay a premium for "clean" power. And renewable energy targets, even ones as soft as those set in June, can only buttress their sales pitch.

Most environmentalists aren't cheering, however, as Canada's hydro producers go on a building binge. Big hydroelectric projects are always controversial. Building large dams leads to flooding of vast tracts of forest, destroying ecosystems, usually at the expense of the indigenous peoples who rely on them. The breakdown of organic matter in flooded areas belies the claims of hydro promoters that such power is "emissions-free."

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Big hydro producers generally insist that the amount of carbon and methane released into the atmosphere from such decomposition is tiny compared with emissions from fossil fuel-sourced electricity. What's more, as Hydro-Québec's website claims, "these emissions are temporary and peak two to four years after the reservoir is filled. During the ensuing decade, carbon-dioxide emissions gradually diminish and return to the levels given off by neighbouring lakes and rivers."

A new study on emissions from the world's one million man-made reservoirs disputes such claims, however. It concludes reservoirs generate far more greenhouse gas than estimated, including about 25-per-cent more methane per unit surface area. Methane is a more potent greenhouse gas than carbon dioxide, about 86 times more powerful than CO2 over a 20-year lifespan. "With the global boom in dam construction, this means the greenhouse gas effect of each new reservoir project will be greater than previously thought – especially in the short term," explained Bridget Deemer, lead author on the study published last week in BioScience.

Most countries don't even report emissions from dam reservoirs as part of their national greenhouse gas inventories, because guidelines set by the Intergovernmental Panel on Climate Change do not call for it. The study's authors suggest that should change – fast.

The study reveals that current methods for calculating GHG emissions from dam reservoirs are wholly inadequate, since they do not capture what's known as bubble-based methane emissions. Such emissions are likely to be relatively high in reservoirs, whose water levels fluctuate much more than in natural lakes.

This is particularly true of hydro reservoirs. Operators such as B.C. Hydro and Hydro-Québec typically let water levels build when market prices for electricity are low, preferring to import cheap U.S. power (often from coal-fired plants). When spot market prices spike, they run their turbines full speed, emptying their reservoirs to export power.

Hydro-Québec does not dispute the BioScience study's findings, but says emissions from its reservoirs are among the world's lowest – 20 to 40 times less than those produced by reservoirs in tropical zones such as the Amazon. The study is less categorical, however, concluding that neither latitude nor age is the best predictor of reservoir emissions.

It may well be that most, though not all, hydropower projects have a lower carbon footprint than natural gas-fired electricity plants. But they are not, as their promoters suggest, emissions-free. And their overall environmental and social impacts, given the ecosystem destruction involved in their construction and their effects on aboriginal lifestyles, may be high.

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The Trudeau government recently approved permits for the $9-billion Site C project, which involves the creation of an 83-kilometre long reservoir up to three times the current width of the Peace River. That is a smaller reservoir than would typically be needed for a hydro project this size, since Site C will reuse water from two older dams upstream. Still, that hardly makes it a "clean" power project.

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