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....
If you push a car uphill turn the wheels and it is held by the curb, it will stay.. tides deposit objects all the time.
How do you know it was a 20lb bag at TBAR? on the bottom? did the money bag break open in the Lewis tumble?
Maybe just a bundle, or the briefcase.. could the surface reverse flow and salinity increase buoyancy? What was the impact of layer and edge flow differentials?
Could it be pushed 10 miles at just over 1/mph then settle on the shoreline.. or snagged and later dredged up?
I don't know, and neither do you..
We do know this, there is evidence that the Columbia has/had a periodic flow reversal well beyond TBAR..