I figured I would give you a short tutorial on making a waterfall look good.
First you want to choose the flatten ground tool and build the ground up behind your waterfall. Once you have your ground level go under game object and then environment.
You will be choosing river water-single long. Just Type River in the filter and it will find it for you. Lay your river starting from the edge of where your rocks are on top straight back.
After you have done this grab the river water single short and lay it horizontal to the river you already laid down overlapping some of the river long.
Now grab the move object tool and move your rivers down until the seams come together. Use the rotate object tool if you need to do so.
Next use the hill gentle to raise the ground up on the edges of you rivers.Now use the valley gentle to sculpt around the edges of your lake.
If you do it right you should get really close to what I have above. You will need to use two rivers to make it look like a lake on top of the waterfall. you can also use the river rapids to make the lake come alive.
Set your camera non routable right over the waterfall and you should only see part of the lake.
To hide any straight lines at the edges of your lake you can use rocks or trees so you won’t see the straight lines. I hope this help some make a more realistic waterfall. It will be hard in the beginning but if you play with it you will come out with something like the one above