Koi, plants etc

Question
I'm putting in a pond this summer, with a waterfall.  15' long by 8' wide. I do live in Michigan. I plan on keeping the pond from freezing over during the cold months.  How deep does a pond have to be for koi to winter over? I haven't decided if I want koi or not but I'm thinking about koi. I might just put gold fish.  I would like to have frogs, snails and red earred slider turtles. Can I have the three in one pond? I've been told that the turtles could winter over in the pond when they get older, is this true? I'm also planning on water lilies, dwarf cattails and other leafy plants...Thanks

Answer
In your area, you probably want at least 4 ft deep for koi.  If you just have goldfish, then a 3' area is probably enough.  For overwintering information, see http://www.fishpondinfo.com/winter.htm
Frogs, snails, turtles, goldfish, and koi can all live in the same pond but some will eat some.  Turtles eat smaller snails, tadpoles, frogs, goldfish, and young koi but some turtles eat more than others.  In a large pond, predation may not even be noticeable.  The goldfish and koi also eat tadpoles. For more on those various animals and plants, see my various pond pages at fishpondinfo.com
Aquatic turtles over winter on the bottom of the pond so like fish, you must make the pond deep enough and keep an opening in the ice.  Good luck!