Thanks to marios21
for the question, that make me to search about it and learn one more thing on the editor
first we paint the area we want with water
secont we add the cliff
Trird we make a path where we want the water to flow. With "Edit cliff tool"
Selected Right click
on the cliff and creat the path
Forth we paint over the cliff with water don't forget to select the same water that used for bottom layer
Fifth now "with smooth tool"
we are smoothing the path we made earlier and use the "Raise/Lower Terrain tool"
to help the smooth process until they join.Dont forget to use small brush
You are done with basic shape,now it's up to you how to edit and decorate