A beaver sometimes swims by the shore in the morning or evening. The pond must be providing enough shelter and sustenance to sustain it. There are green plants along the shore from spring through autumn, and plenty of trees to cut and set aside for the winter. The water must be deep enough to cover the entrance to the lodge and to allow a dive when disturbed.
And the dam is already there. No need to build another. How much time and effort must that save for beavers? The human subsidy frees them to forage, and maybe even to have a little down time. Do beavers ever take a break, or are they really always busy? I’m guessing they rarely stop working because there is no telling how long the ice will cut off their food supply. There is no telling how much food needs to be stored or consumed to build up body fat. Beavers sometimes have too little food in the winter, and they starve. Life can be harsh.
But a ready-made dam probably increases the odds of success. So do all the adaptations of beavers for living in a pond: teeth for cutting plants for food and construction, webbed feet for swimming, modified myoglobin to store oxygen while diving, and the layers of fur that repel water and maintain insulation year round. Beavers have to occupy places that allow their adaptations to work. This pond is one such habitat. And when it is sufficiently dark and there is no ice, there are beavers gliding through the pond, their eyes, ears and nostrils just above the surface. Adaptation indeed.