Contaminants Can Flow Up Waterfalls, Say Physicists

They found that the leaves (and also chalk powder) were able to navigate upstream if the waterfall was less than about a centimetre in height. “For distances of the order of 1 cm or less, some of the floating particles eventually start to “climb up the stream”,” they say.

This apparently refers to particulates; solutes could presumably do this via diffusion to some extent without setting up a counterflow.