Gems and Crystals

If they are the flat, smooth stones you could use them to make runes, mark the boundaries of a ritual circle, or even use them as a means to connect to the river and the water element its connected to. You could also polish them and use them as worry stones (smooth stones with a small indent that are used to help relieve anxiety).

Ritual circles was what I was thinking but I wasn’t sure, so that’s good to know. Thank you.

From the little I know about the topic of crystals in general, I think that river stones can be a useful part of your altar(for the part of the pentagram that's supposed to represent water), for fairy magick, and magick relating to the element of water.