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It is estimated that when Europeans first came to North America, there were anywhere between 60 million and 400 million beavers, occupying about 6 million square miles of habitat

Although the beavers were almost completely wiped out from most areas, their dams and pools remained for many years. But they did not remain in the same state as they would had the beavers also remained. Without the beavers there to maintain the dams, some dams broke apart, and some ponds gradually changed into marshes, while others filled in completely with sediment and new forest trees grew.