I live in Colorado at the foothills of the mountains. I don't think the high has been out of the thirties all week. I have a 14,000 gallon pond that is 6 foot deep. It has a Pergola over it that is covered with greenhouse film. Right now the water temp is 45deg. I stopped feeding at 50deg although they would readily eat now if I offered it (they are up at the top begging). I actually tried feeding them once a couple weeks ago when the temp was 47deg and they ate like I hadn't fed them in a year.
I do have a heating system on order from one of the dealers that will keep my Koi toasty all year.
Daniel