Rain falling in a watershed drains into rivers in the valley.
One word can explain why any river exists on Earth—gravity. You've heard that "water seeks its own level," but really water is seeking the center of the Earth, just like everything else. In practical terms, water generally seeks to flow to the oceans, which are at sea level. So, no matter where on Earth water is, it tries to flow downhill. With the Earth being a very unlevel place, water ends up occupying the valleys and depressions in the landscape as rivers and lakes.