bz,
You are correct that a siphon is totally run by gravity. When I said that a siphon would flow more water through a given pipe size than gravity could, I was comparing a siphon to a standpipe system. You are again correct that this is because of the added head pressure that a siphon system has, and a standpipe does not.

Why do I think this is better? Well, my yet to be built pond will be on a small spring fed creek. Most of the time my two 12" siphons will be acting like a spillway with the water just trickling through. But when the terd-floater storms come through and the water rises, it will go into siphon mode and hopefully be able to handle all of my 320 acre watershed before it gets up to the emergency spillway, also known as my driveway. I don't like the look and constant noise of a standpipe and all of the trash that collect around it.

More good reasons for a siphon...It draws water from the bottom of the pond, pulling the oxygen-rich water down from the surface. It doesn't require a pipe through the base of the dam where a leak is expensive at best and dangerous at worst. If I ever need to drain the pond, all I have to do is cap off the air vent and wait about a week.

Hey Moe, I'm trying to think but nuthin's happening!