Simple, Jo Weber said Duane threw the paper bag over the bridge near the Red Carpet Inn, or when Duane (aka Cooper) parked near Tbar taking a shovel with him and returning and driving off. I don't know why everyone makes this so difficult
Even if the money bag floated on the Washougal and Columbia water surfaces all the way from its splash down point, it would not end up at Tina Bar. It would end up on the west side of the Columbia River when it went around that curve from west to north. And there is no reasonable way for it to get to the east side of the river and Tina Bar.
The shipping channel at Tina Bar is on the west side of the river is dredged to a 40 foot depth. This depth is presumably measured from a nominal surface elevation of the river. And that surface is probably only about two or three feet above sea level.
Eric recently stated in one of his videos that the bottom of the Columbia drops off rapidly just a few feet off shore from Tina Bar. There is no way that the money bag could go uphill if it was under water.
So my guess is that the money bag, maybe with Cooper attached, came from a location very close to Tina Bar and was actually moving downhill at that time. This means that it arrived during a substantial flooding episode and started several feet (maybe 10 or 12) higher than the elevation at which it was found.