Theo - 10-15 degrees would be significant. I'll have to research it and try to find some hard data before I reply to DEP.

Jeff - I would be interested in seeing a sketch or drawing of your system. I've been debating whether to use a standpipe or siphon. Your solution may be a third option.

Ideally the pond overflow temperature would be the same as the temperature of normal watershed runoff.

Thanks to all.