Riverine ecosystems and rapids like this would be lost. Photo by Mark Clement
Hydroelectric dams typically have a life expectancy of ~100 years, during which their reservoirs will continuously emit CO2 (carbon) and particularly CH4 (methane), even in temperate and boreal climates. In fact, Ontario Power Generation is in the process of rehabilitating several of its 66 hydroelectric facilities in Ontario, including the 117-year-old Kakabeka Falls Generating Station. Unfortunately, this means these facilities will continue to emit copious amounts of methane for another ~100 years.