with the bag untouched by anything on the bottom, able to move freely would have to have more reverse flow than the natural flow..it could never make any backwards progress fighting the natural current in less time.
the bag would simply move back with the flow of the current erasing any backwards motion. you can push a car up a hill, but if you stop, the car will just roll back...so, if the current was strong enough to go against the natural flow for 8 hours, it then has 16 to remove what it lost....