This thread might already be dead but I will add my .02. I use a hydration bag in the supplied pocket in both my Gregory packs. A full liter of water is the heaviest single item in my pack and I normally carry two, since I hike in the SE. I use a gravity filter and a camelback hydolink to fill the hydration bag. No need to access the bag on the trail. I do carry a small disposables water bottle for flavored drinks. In my old pack I would put it in the lid or just under the lid but I found that 2-4 lbs. in the top would throw off my center of gravity