Prophecy (1979) on Netflix

The idealistic Dr. Robert Verner is invited by his friend Vic to travel to the Maine with his wife Maggie working for the Environmental Protection Agency to prepare a report about the dispute between Indians "Opies" (meaning the original people) and the lumberjacks. Maggie is pregnant but she has not told to Robert since he does not want to have children. Robert and Maggie are welcomed by the director of the paper mill Bethel Isely that drives them to their cabin in the woods. However they find an Indian blockage of the road and witness a conflict between the leader of the Opies John Hawks and Isely and his men. On the next morning, Robert goes fishing and he sees a huge salmon in the river. Then they meet with John Hawk and his wife Ramona and they tell that their people are ill and most of their babies are born with deformation. They also hear about the legendary Katahdin and visit the Garden of Eden of the Opies, where Robert finds weird plants with roots on the surface and a huge tadpole. Robert continues to investigate and discovers that the industry is using methyl mercury in the process that is causing the mutagen in the environment.