That's a really good site to build a waterfall on. I'm sure it will look great when you're done with it. I just read your question and figured I'd try to help.
Let's talk waterfalls....... The sheet of water that flows over a waterfall is called a "weir." There are calculations that will let you determine how many gallons per minute of flow will be required to get a weir of a certain width and thickness. For example, if you had a waterfall weir that was let's say 6" to 11" wide, to have enough flow to make that weir 1" thick, you'd need to have 3300 gallons per minute of flow through that waterfall opening. If you had an opening of let's say 12" to 15" and you wanted your weir to be 1" thick, you'd need to have 4500 GPM of flow to acheive it. There are charts in pond building books (one of mine has one, but that won't do you any good) that lay out how much flow you'll need to achieve a weir of certain thickness given a particular width. Here's the web address of a chart you can look at for weir size calculations. >>
Here's a rough guide that will get you started. This will at least give you an idea of what you'll need as far as pump capacity.
Waterfall Weir Rates
Width of Weir Thickness of Water
(GPH/GPM per ft) Smooth Weir Stone/rough Weir
360/6 1/4" 3/16"
600/10 3/8" 5/16"
900/15 1/2" 3/8"
1,680/28 3/4" 9/16"
2,400/40 1" 3/4"
4,500/75 1.5" 1.25"
6,720/112 2" 1.75"
Once you figure out how much flow you're going to need to get your weir looking the way you want it to, simply select a high head pump (one that's made to elevate water great distances) that's rated to flow the amount of water that you want to move.