I would say that these are not sharply defined categories, there is more a like a smooth continuum from "fast-flowing stream" to "rapids" to "small waterfall" e

I would say that these are not sharply defined categories, there is more a like a smooth continuum from "fast-flowing stream" to "rapids" to "small waterfall" etc. And this is a good environment in which evolution could progressively improve rock-holding and climbing ability.