onto the guy's dinghy. It kind of looked like the stern hit bottom and the rest of the boat was pushed downstream over it.

Have you ever read the Log of the White Knuckles? The guy tells about anchoring his Hunter 25 in a river (Mississippi?) and the current was at/above hull speed. He had a dilemma, which was aggrevated when a forked tree swept down over his anchor line. He just made it out of that one by the skin of his teeth!