The short answer: Site C has more significant adverse environmental effects than any project ever examined in the history of Canada’s Environmental Assessment Act.

The Long Answer

The adverse environmental impacts of the Site C dam include:

• destruction of habitat for more than 100 species already vulnerable to extinction, including bird, plant, butterfly, bee and mammal species—this at a time when scientists warn we are facing a biodiversity crisis.
• extensive loss of very old forest including old-growth boreal forests, key for caribou and other species, along 100 km of the valley inside the flood zone
• destruction of ancient tufa seeps
• destruction of key animal and bird breeding areas like the famous Watson Slough
• desiccation of the downstream Peace Athabasca Delta and UNESCO site, Canada’s largest national park Wood Buffalo Nat’l Park
• impacts on fish and other species in or dependent on the river, via interruption of river flow and methylmercury concentration

Sources:
https://www.theglobeandmail.com/news/british-columbia/bc-hydro-understating-site-c-dams-impact-environment-canada-says/article15991478/
https://www.hilltimes.com/2019/05/06/198550/198550
https://thenarwhal.ca/site-c-far-from-clean-green-finds-new-ubc-report/
https://www.sitecproject.com/document-library/environmental-assessment-reports

NOTE! The federal government of Stephen Harper issued the environmental permits for Site C during the election writ period, which is highly irregular. The Trudeau government was well within its rights to overturn those irregular permits when it came to power, but it chose not to. Trudeau, who is clearly close to SNC-Lavalin, has stated publicly that he wants to see scores more dams ‘across the North” as part of the federal government’s clean energy initiatives. On whose land, and on melting permafrost? When dams are more environmentally damaging than the alternatives?