I don't mean to be insensitive, but how do you drown in horizontally-moving water? Isn't it the same as swimming in regular water?
Rivers are extremely dangerous to swim in because of their erratic currents. Get too far from the shore and you'll get sucked right under and won't come back up.
There's a river at a beach in Maine that I used to vacation at, and several people are known to have drowned from swimming in it. I've actually swam in the more shallow parts of that river and could barely stay on my feet from the current throwing my legs all over the place.