There are certain immutable truths about the outdoors, among them: If you don’t ask a first-time tentmate prior to a trip whether he snores, he will…loudly. If you decide to cut pack weight by bringing a lighter bag or insulating layer, it will be freezing. And add this contemporary truth: If you don’t use a pack with a convenient hydration system, you’ll drink less water, less frequently, than you should.

And, leaving aside the hyperbolic marketing phrase “hydrate or die,” we all know that staying hydrated is a good thing.

Going with the flow

Today’s hydration systems are composed of two very simple parts: a tough, flexible sack-like bladder for containing water, and a drinking tube that connects it to your parched lips. The bladder is usually placed inside your backpack, with the tube clipped to a shoulder strap or other convenient point for ready one-hand access. A valve on the end of the tube prevents water from dribbling out when not in use.

This bladder-and-hose system has a distinct advantage over water bottles. Having a drinking tube ever at the ready means that you no longer have to stop and open a bottle every time you want to drink. No more fumbling reach-around while wearing your pack to grab a water bottle that always seems to be just out of reach. No more asking your friends to replace it once you’re done. And no more worries about the bottle falling out of a loose pocket.

Plus, water is heavy, and a bladder rides against the middle of your spine, exactly where you want weight in a pack.

Down the tubes?

But a full bladder is not the hydration panacea it may seem. Overall, I’ve logged thousands of miles with hydration packs and experienced few failures, but there are some drawbacks to consider:

Flexible bladders are more susceptible to breaking than a sturdy, Nalgene-type plastic bottle. Refilling a floppy container is a challenge at times, and retrieving it from inside your pack in the first place can be a hassle. In sub-freezing conditions, a hose and valve are more likely to ice up than a wide-mouth water bottle. And bladders are more complicated to clean than bottles (see sidebar).

However, most of these are minor, and easily addressable, concerns. While I’ve sat on a pack and burst the bladder inside by pressing something sharp and hard against it, careful loading of the pack readily negates this risk.

Fast-moving streams and shallow ponds can make refilling a bladder difficult. On multi-day trips, I like having both in-pack hydration and a bottle; the latter may be empty on the trail, but can be used to refill a bladder, and is more convenient in camp.

In many packs the bladder rests inside a sleeve deep in the main compartment, requiring you to unload some contents to access it. Some styles keep the bladder in a separate pocket, usually between the back pad and frame, where it’s easily removed and replaced—a nice feature.

In cold temperatures, small amounts of water in the hose can quickly freeze up. To prevent this, blow the water out of the hose and into the bladder, which is less likely to freeze because it’s close to your body, and tuck the tube inside your jacket. Using this technique, I’ve rarely had problems with temperatures in the 20s.

Many packs designed for winter activities often feature a hose insulated with neoprene, which sometimes zips into a shoulder strap. I’ve used such packs in temps near zero degrees without the hose freezing as long as the pack’s on my body.

Packing it in

As you start shopping for a hydration system, beware more slippery marketing speak: “Hydration packs” are those that come with a bladder and hose, whereas “hydration-compatible” packs have an inside sleeve for a bladder and a “port,” or hole, for the hose to exit the pack—but you have to supply the bladder.

Bladders come in a variety of sizes, with most ranging in volume from one to three liters. Many styles can be swapped between packs, though some sleeves won’t accommodate bladders larger than two liters. Packs fall into one of three categories, with dozens of models in each:

Hydration packs for aerobic sports.

Designed for activities such as trail running and mountain biking, they give priority to low weight and bulk, and sleek, unobtrusive design. They have little cargo capacity, and generally fit no more than water, snack food, and a light jacket—good for mild-weather outings of less than a full day.

Packs for full-day outings.

Intended for trips with potentially variable weather, or that demand extra clothing and gear, these pack styles vary greatly in price, cargo capacity, weight, design, and specialized features for activities like rock climbing, mountaineering, or backcountry snow sports. Leaner, lighter models weigh about two pounds or less empty, with enough support to handle loads of up to approximately 20 pounds. More supportive packs typically weigh in around three to four pounds, can comfortably haul 25 to 30 pounds or more, and have the stiffness to carry heavy, awkward gear like skis or snowshoes.

Multi-day backpacks.

Most packs designed for extended adventuring are usually hydration-compatible only. As with daypacks, these vary greatly in price, capacity, organization, how much weight they can handle, and features.

As with any pack, the most important feature to consider is the one that’s least visible: carrying comfort. Begin by finding a pack that fits your torso length, and allows you to carry the majority of weight on your hips rather than your shoulders. Most smaller packs aren’t adjustable, but some come in two or three sizes. Next consider how much weight you’ll carry, load up the pack, and walk around the store wearing different models to compare comfort and fit.

Overall, pack users and designers have gotten a lot smarter about hydration. I’ll drink to that.

Courtesy of the Appalachian Mountain Club