Then why is it called a trap?
"Outlet elbow
The elbow on the outlet end of the pipe serves three purposes. It acts as an aerator so the anoxic bottom water is aerated before it enters the stream, as an energy dissipator so a deep plunge pool doesn't develop and as an air lock so siphoning will begin at lower water levels (low flows) and will continue until air enters the upstream air vent."
See
http://extension.missouri.edu/explore/agguides/agengin/g01530.htm for reference
I think their adjustable intake vent is pretty neat too.
The article also brings up a good point about the intake freezing. What if just water in the air vent freezes, forming a plug? Couldn't that potentially drain the pond if a siphon started?. It seems as though it would be better that the vent was above the bottom of the high point in the overflow pipe.
