We have 8" siphons on a couple ponds. If you don't have to worry about freezing, the automatic one is a great system.

http://www.ponddampiping.com/syphon1.html

One advantage of the ones we have with a valve is that you can start them in advance of a big storm to help prevent possible overflow. A disadvantage is you'll lie awake nights during big storms wondering if you should have started the siphon or you'll drain down in anticipation of a storm that doesn't materialize.

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