First of all make a deeper Ground.
Press on the River tool. The First thing what you do with this tool is to set points.
After you set the points it have to look like this. Then just set the Water height to a higher setting as the deep ground. At me Deep Ground = 0 , Water height = 4
Then it looks a little bit like this but thats not really beautiful to change this just change something on the ground where the water is like the texture.
Then it is finished looks now much better. GOOD LUCK!