Act responsibly and use common sense and you more than likely won't die. A flooded river is very powerful and personally I don't thing a pfd will do a lot for you in a river flowing out of its banks. It will prolong your death, not prevent it. This is my opinion not a fact
PFD=Personal
Flotation Device
Life Jacket is a BS term. A jacket won't save you're life. It just helps you float.
For starters, wear the damn thing. It doesn't matter how good of a swimmer you are when you get run over by a jet ski.
Second of all, it only makes you float. You still have to get yourself out of harry situations if you happen to be unlucky. If you've never swam in swift water, you probably shouldn't be out on a river near or above flood stage.
Strainers are a HUGE hazard when rivers flood. What may look like a harmless pile of debris can kill you real quick.
If you're worried, just paddle flat calm water, wear your PFD, and you'll probably be good to go.