I would guess that coins and other artifacts that were in houses and other buildings would be scattered along the water route as the buildings broke apart and their contents fell into the water. Just like ocean waves (especially during storms) can wash objects ashore (shipwreck items), I'm sure the force of a rain-swollen, raging river can do the same thing. In a nutshell, I would guess that there are coins, jewelry, and all manner of household items scattered and buried the entire length of any river that has ever flooded and carried buildings and their contents somewhere farther downstream.
Maybe there are historical photos that show the location(s) of some of these buildings and where they came to rest after being carried downstream. That, at least, would be a starting point.
Harley-Dog