Yeah, one of the oldest known methods is basically using mud from a riverbed to scrub. It exfoliates better than a lot of modern things (like those damned nylon scrubbies) because it’s basically pumice stone dust. The friction from the large surface area does a decent job at carrying grime away. And since the riverbed is made up of sandy particles that are large enough to sink and not get carried away, nothing “dusty” gets left behind afterwards. So you actually come away surprisingly clean.

I’ve used it while camping before, and can confirm it works. The big issue is that you can’t get your hair clean with this method, so you’ll still end up smelling like wet dog unless you have something to cut the grease in your hair.