I have been considering a Lowepro Flipside 400. The 400 is the largest version of this series, but it is not too big and heavy. I am not sure how much food and water you carry, but this bag is big enough to carry two camera bodies, several lenses, and lots of accessories including a tripod attached to the outside. Water bottles go in pockets on the outside, and you can attach add-on pouches for energy bars, etc., if they don't fit inside.
A neat feature is that the bag opens from the side that is against your back. That allows you to slip out of the shoulder straps, slide the bag to the front using the waist belt, and open it like a work platform without setting it down in the snow or dirt. The 400 version has a large, reinforced belt to hold it firmly in that position. You can change lenses and filters without exposing it to the snow or dirt. Also, when you take it off and set it down, the back opens without letting in snow or dirt on any side.
See one at: http://www.bhphotovideo.com/c/product/5 ... ml#reviews
The Lowepro website has similar info. The video is cool, showing all the features. See the reviews as well.