Beaver's can scare the bejesus out of people with that tail slap.  In 2013 they got my dog pretty good.  We were sitting around shore at dusk on a quiet evening and then "Kerplunk!".  Luna bolted upright and snorted at the lake.  :)

This year on Camel Lake after dark I was laying in the hammock and had one do about 3 kerplunks before moving on.  They are assertive creatures.