Streamflow moves water as part of the natural water cycle.

Streamflow

 Streamflow is the water flowing in the creeks, streams, and rivers that you see in the low-lying parts of the landscape. After the rain falls, most of it runs off the land downhill, towards rivers and the oceans.

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. So, no matter where on Earth water is, it tries to flow downhill. With the Earth being a very unlevel place, water collects in rivers, which are the highways for precipitation to eventually get back to the oceans.

A watershed is the area of land where all of the precipitation that falls in it drains off and goes to the same point in a river. Think of a big footprint, pointing downill, in the mud on a steep hillside. Whatever water falls onto the area of the footprint will flow down to the toe and maybe flow out (as a "river"). The whole footprint is the "watershed" for the point where water leaves the footprint.