Beavers are amazing creatures. The skill and complexity involved in constructing their dams is incredible.
The construction of a beaver dam is probably just an emergent property of the interactions among individual beavers following simple rules rather than a result of a detailed preconceived plan. I don't think beavers have a blueprint or a mental image of the final dam; their behavior is guided by instincts and a set of rules that lead to the emergent structure.