We have confirmation. A 2" valve will stop an 8" siphon with lots of drop. We stopped the siphon this evening. I recorded some video and pics with my Blackberry. I'll upload it within the next few days.

An interesting and logical observation was made. One of my nephews was at the discharge end and another pulled the leash to open the valve (thought it might have a vacuum at the valve - and it did). The nephew at the discharge end said that all of a sudden, the water shot out two feet further than it had been, before stopping. I guessing that when the valve opened and broke the suction, the water was able to run out faster because it didn't have to pull the water up the other side of the pond.

It has been below freezing several of the nights it has been draining and I was worried about losing fish. To my knowledge, we had 6 - 10 channel cats go through the siphon and no other fish. Apparently, being sucked through the pipe, around a couple 45 corners, and shot out into a metal cage is not a fun trip as many were dead when found. The water turbulence and freezing temps may have contributed to that too.

[Linked Image from i108.photobucket.com]