Choosing a Pump for a Waterfall

First, determine the height and width of your waterfall and choose a pump that will give you 100 gallons of flow for each inch of water fall width. This means that if you had a 6' high waterfall that was 8" wide, you would select a pump that will supply 800 gph at 5' pumping height. (100 gph for every inch of waterfall width) Select a pump that will pump a minimum of 1/2 of your ponds total volume every hour. Remember to calculate this using the GPH flow rate at the pumping height you'll be using in your pond, not the flow rate at 1'. 

How to Calculate the number of gallons  of water in your pond.

For a rectangular or square pond
Length (ft) x Width (ft) x Average Depth (ft) x 7.5 = volume of water
ie a pond 4' x 8' x 2' avg depth would be 480 gallons

For a Round Pond to get the cubic feet and multiply by 7.5 to get gallons (volume)
1e.  a pond 10' in diameter and 2' average depth
(5' x 5') x 2' average depth x 3.14 x 7.5 = 1,177.5 gal.