The "shelf" is just a strip of silnylon about 2" wide at the bottom of the pack which in theory would keep the bottom of a hydration bladder from slipping forward. A hydration bladder, in my opinion, takes up too much room in this pack so when I use my 2 liter zip top platy, I tuck it behind the sitlite pad in the pad sleeve. With a full load of gear, I consider this arrangement to be uncomfortable & use water bottles in the side pockets instead. There is no clip to hold the top of a hydration bladder. I only use a hydration bladder with this pack when day hiking & only then, in the manner described above.