Blender Beginner Tutorial: Simple Forest | Learn how to use Blender. 

There is another easy way to create the river. Instead of deleting the extra faces that we had created, you could create a new river material inside of floor material properties, go into edit move, select the faces, and click 'assign' on the river material in the material properties.

Despite the fact I knew most of the stuff, it's still a great Tutorial, especially for beginners who are new to blender!