86:5 One may stand on a river bank on Shabbos and wash his hands in the river. It is not problematic that one carries water on his hands from the river to the shore because the river is a carmelis and the bank is also a carmelis, and one is permitted to transport fewer than four cubits from one carmelis to another. One must, however, be careful to dry his hands carefully before he walks four cubits.
86:6 One may clean his hands by rubbing them with crushed grain even though his hands are wet so long as he doesn't put water directly on the grain itself (because of the prohibition against kneading on Shabbos - see Mishnah Brurah 326:28). One may not, however, rub his hands with salt - and even more so he may not rub a bar of soap - because doing so softens these substances.