There are a few areas where beaver dams spawn on the Fjordur map of Ark Survival Evolved. Luckily these spawns are probably not too far from your location. Oddly, the Castoroides themselves are not spawning on my single player map at the moment. Hopefully, they fix this beaver spawn issue soon. I would really like to tame a beaver! I've also seen that dams have disappeared (de-spawned?) on my map for no reason I can determine.
Fjordur Beaver Dam Coordinates