First make sure that you have a textures with the name "scrollwater". If you don't have a texture with that name open Wally and rename it, cause the texture you are gonna use for your waterfall must have the word "scroll" in it's texture name inside the .wad file.
Now find the texture and make a brush. Link Then open your Texture application tool and stretch the texture a bit Link
When you are done with this mark the brush and press CTRL+T and choose func_conveyer and use these settings:


Render mode - Texture - some light
FX ammount - 100, 150 or less
Convoyer speed - 150, 200, 300
Angle - Down (Also known as Yaw depends of the FGD file)

Then press Flags and check both of them. Link Now go and compile your map .