You should really only have one back-pack strap on.  Try sometime, to 'unload' a full backpack, both straps on, in the water!  Many, many years ago, I was down on the Kenai, just south of Turnagain pass.  I was hiking up a trail to the east of the hiway, when I came to a stream, fairly wide (20-25 feet or so), not too deep (11/2 - 2 feet ).  There was a man standing there, head down on his chest.  I stopped and waited a moment, and then asked if he was OK.  He related to me, that they (the other men had already left)  had just retrieved the body of one of his best friends from this same stream, he had drowned trying to cross!  He turned to me, quite angry (not at me).  "He knew better!" the man exclaimed.  "He knew not to have both shoulder straps on when crossing  a stream.  We found him face down, he couldn't get rid of his pack, and it killed him!"  This was long enough ago, there weren't hip straps or sternum straps on backpacks, and he still was unable to dump his pack!