oldsoldier, I can't speak for the cold climates but as long as you get the turbidity (cloudy water) relativity clear it will kill everything in the water but pesticides,mercury etc. You can clear the water (which I know you know) by using a bandanna, tee shirt, scoop the upper part of the water or just let the water stand for a while to get the water clear. It just needs to expose everything in the water with the UV light to kill it, muddy water acts like a shade in the sun and you have to remove the shade so they can't hide. I have heard people who have kept the pens in the pocket and batteries in the pocket have had success, I have only tried it in warmer temps to about 30 degrees.