Jersey, I gotta ask. How is it that you figure a siphon can move water faster than gravity. A siphon is totally run by gravity and its potential can only be the difference in the height of the water from the surface in the pond to the outlet of the syphon. Maybe I don't understand what your system looks like and the placement of this air vent. It is true that an air vent at the highest point of the syphon tube will make a pipe no different from a spillway until the water covers the vent. Is that what you mean? If so, then it is true that the syphon can result in much higher flow because the height difference between inlet and outlet for a spillway may be only a fraction of an inch while the end of your syphon tube could be many feet below the water height in the pond. I'm new to pond management and my pond does not have a dam or overflow need. It would seem that what you describe would be a good way to accomplish overflow that is just a trickle when pond level is relatively close to what you want and then automatically goes into syphon mode so that flow goes up sharply if the level suddenly gets too high. Ingenious, but why do you do this unless you need to have the pipe inlet on the pond side always below the water surface but don't want to drain down below the highest point of the pipe. Is this a common need for ponds?

Gotta get back to fishin!