Cliff,

Do you mean the siphon took the pond down further than anticipated? or maybe it leaked around it?

If the former, the lowest a siphon could lower the pond is its depth on the pond side. But it could only do so if the siphon outlet is at or below the elevation of the pond side siphon entrance. Otherwise ... the lowest the siphon could lower the pond is the elevation of the outlet.

If the level of the pond is below the level of the outlet ... then there must be some other problem.

It isn't what we don't know that gives us trouble, it's what we know that ain't so - Will Rogers