Just to see what would happen, I went and put a copy of a plaster face in TufStone into a creek near where I work. The water's pretty brown, (tannic acid?) and runs shallow and rapid where I put it in.

I wedged it in gravel so that the water just hit at eye level. This was on a Friday afternoon. On Tuesday, the water had eaten through, and the top of the forehead was a bit further downstream.

Another shot, just of the bottom half. The green fibres are algae.
What the cast looked like originally.