To empty a Beaver dam(always completely empty to allow more to spawn), open Iventory of dam, then immediately close it and jump on a semi quick tame(preferably a bird). The beavers will now follow u anywhere. Lead them far away, or over a ledge then return and empty tha dam. *this has to be done for each individual dam.
Outstanding wood collectors. Mine has 770 weight and can carry 3K wood plus rider. Holds wood at 1/5 weight and can be used as mobile smithy. This teamed with a doed can construct a base in minutes. Must have.