Tyler Ludens wrote:How large does a stone construction need to be, and how constructed, to moderate winter temperatures? I'm thinking of some kind of sun trap thingy on the north end of a small pond, and would love to be able to keep the winter low above freezing in that locale for growing some citrus and/or possibly a hardy avocado. Our winter temps get down to around 15 F. The area is backed to the north by a shed and a 2500 gallon rain tank. Any ideas would be appreciated. 
I'm doing something like that, testing it the next months, have no idea if it will work, just make sure the sun hits the rocks and the cold wind doesn't, also be sure that it doesn't become a cold air trap also, cold air should flow easily above the lake water, cold air was something I learned in Paul Wheaton podcast, he's right about it flowing in the landscape and giving trouble whenever it gets trapped (usually in low lands).
Trees also help but don't put them near the water because their roots will damage your pond and their leaves will fill it up and make it smell very bad.
Rocks just make it less could during the hours with lowest temperatures thus not freezing the water, if it's a small pond I would also cover when temperatures get very low, maybe let the sun hit it during the day and cover it during the night.
Last year my pond was smaller and had maybe half the number of rocks on a similar configuration, it frozen just a little on the winter, one or two hours after the sun rised all the ice was gone
