Sorry, it seemed really obvious to me.  It is a lowhead dam.  While you can hear it a ways ahead, you cannot see it until you are almost on top of it.  If you go over it, you will be caught in hydraulics and most likely drown.  There are no trespassing signs on the other side of the river, so this is where you must portage your kayak around the dam.  There are no markings or signs in the river warning you, so this is an extremely dangerous place.