In this picture, I have made a very (VERY) simple cliff. Now the red circles show that I have made some cliff tops on the sides of the waterfall that jet out of the structure. "I (you xD) wanted to make some variation in the cliff by making it seem like one part goes farther than the other, but, in the green circles, I (nisage) didn't remove the cliff parts next to the part I want to be farther out.
In this picture, in the green circles, I have made the cliff height correct by making it match the size of the red circle part.
Get it? If you don't understand what I wrote, then try to make your cliffs match my cliff.