@halcy @gargron the rings of saturn are mostly ice, so does that count as a frozen river? Liquid water would be pretty hard to keep liquid in a stable orbit, as it'd have to be outside the atmosphere of the planet, and then there'd not be much else to hold on the heat.

Maybe if the planet was close enough to a star, so the energy from the star kept the water hot enough to be liquid? ... But then it would probably be scattered by solar wind pretty fast :/