Armariel doesn't see any undead on the dam, or signs of them. The logs have some algae growing on them underwater, and the top appears somewhat weathered. He also doesn't notice any tool marks.

Some of the logs used are extremely big, though.

(Nature 19) Armariel recalls that, far from civilized areas, large mammals with extremely tough waterpoof fur and large, flat tails are known for gnawing through trees and using the trunks to build large dams that conceal semi-aquatic dens. They're call Fortress Beavers, and their hides are extremely useful in crafting warm clothing and enchanted hide armors, but they can be tough to bring down and can hold their own against most predators.