The Delta has been discontinued. If you're looking for something new, check out the best hydration packs for 2021.
Historic Range: $44.95-$44.99
The Mountainsmith Delta is a basic, no frills hydration reservoir carrier. This small pack will be nearly stuffed when carrying a full 2 liter/70 oz hydration reservoir.
The pack has two pockets. The outermost pocket is a very narrow but long (running 80% of the length of the pack) and is suitable for carrying small items like a flashlight, multitool, or rolled up very lightweight shell. The larger pocket has two sleeves: One sleeve is for the reservoir and the other is for whatever else you can fit in the pack. There isn't room for much.
Also inside at the top of the pack is a small zippered interior pocket which can be put to use carrying essential identification, car keys, and maybe a very small pocket trial guide, map, or first aid kit. Two velcro loops are also to be found inside the inner pocket along one side. Beats me what you could use them for, though, as they aren't large enough to secure anything with more than 1/2" diameter or so. I'd say that maybe you could fasten an ice ax or collapsible pole to your pack here, but the zipper at the top of the pocket which secures your items inside the pack would interfere with anything that can't fit entirely within the pocket.
Which leads me to my only real complaint about the pack: There is no port for the hydration reservoir's tube. The tube must be snaked out through the zipper meaning that the pack can never be completely sealed when you intend to use it for its primary purpose: hydration!
Also, the zipper zips from your right shoulder to your left, so you must snake the tube over your left shoulder.
The pack carries comfortably and securely at the top of the back between my shoulders. I'm 6 feet, 190 lbs with broad shoulders and a deep chest, and this pack fits almost perfectly. In fact, I hardly feel like I'm wearing it at all. My only complaint is that if the reservoir is filled to the brim, the pack will bulge a bit and ride my spine a little too closely.
So keeping those things in mind, this is a good, very lightweight pack that does an adequate job of carrying a 2L hydration reservoir plus a few essentials. A few more inches of storage wouldn't go awry especially as the pockets almost mock you with their deceptively tiny volume after filling your reservoir. Don't expect more from it, and you won't be disappointed.
Size: 150 cubic inches, 2 L/70 oz hydration reservior
Number of Pockets: 2
Height of Owner: 6 feet