Sorry it's late, I mean GPH. Also waterfall width to be 12". Finding I need more GPH! Maybe 1800.

My pond is just about exactly your size. I just got a Pondmaster Proline, Hydrive/submersible pump, 3200. It seems to have the right power.

Just kep in mind that GPH is rated right at the pump output. The more plumbing, specially turns will greatly reduce output. Also the higher the height of the output the less output.

The most important thing, besides turn the volume over at least once an hour, with sizing pumps is how wide your waterfall, or weir, is and the width of the stream. Guess a minnimum of 1200 gph per 12 inches of weir for a light but solid flow. 1800 and more for a thicker flow and so on.

Make sure your skimmer can handle your desired flow, if you get too big of a pump it can't keep up and it can burn your pump out. Or literally it will be too big to fit into the skimmer.

Another basic rule of thumb is to have the skimmer placed as close to across the pond as you can. You can run into circulation problems if it is too close to the fall.

Also as billinpa said, make sure to take overall head into consideration, 10 ft. lateral = 1 ft. vertical head and too many fittings, especially check valves, can cut down gph. Compare with the pump or get talk to a local specialist.

I have a video on photobucket of my waterfall look for Lefty7_photos, If I knew how to get video in message I would put it in.I have a Pondmaster Hydrive 3200gal pump with around 5ft rise.