Nope, got plenty of Rivers around me, not a single one has a beach, and if you're going to play smart Alec, the beach in BG3 is sandy. Beaches near rivers are often muddy or soft. Not my idea of a beach. The only thing missing from BG3's beech is Palm trees and a hotel resort.
Sorry, I was being a little snarky. I think you mean beaches in the sense of sand that is the product of Parrot Fish and others who eat corral as a form of sustenance and then they poop out tiny little bits of it digested which then becomes the super fine sand commonly found on Ocean beaches. That's right. You are lying on a giant pile of Parrot Fish poop.
But there are many ways that rocks can be ground down to make sand. Albeit not so fine as the sand found on many beaches. The regular flow of water can cause the grinding, and as the river changes course, gets dammed up it leaves those areas exposed. Living things in freshwater also sometimes ingest small rocks or bits of them and poop them out.
Its funny to think our world looks the way it does because of all our incessant pooping.