Your camelbak tube will freeze hard unless you route it where it's warmed by body heat.
I blow the water from the tube back into the bladder when I'm done drinking. Then tuck the valve inside a pocket or zipper to keep it from freezing as residual water in the tube will end up in it. If it's real cold, you can wrap something around the bladder with a hand warmer in it but keeping the valve from freezing is the main thing.
