Rivers form from little trickles of water that get bigger and bigger (more voluminous) over time. And where do these tickles come from? Read Viktor Schauberger's works. He studied water for most of his life having grown up with a father that was a forest manager in Austria. His books of info are incredible.
If no one is bothered to research his work, the original trickles of water are formed when you have an overgrowth of grass, reeds and other undergrowth. This allows a temperature differential between ground level and the top of the grass/undergrowth. This temp differential allows moisture to form on the ground, which turns into standing water due to the volume and causes this water to find its way downhill, joining with other pockets of water. A trickle turns into a stream and over time, a river. The soil contributes to this process by being more of a clay type soil that does not allow the water to dissipate into the ground easily. If it does dissipate into the ground, it will do so until the ground becomes water logged.