This Hydro Flask hydration pack kept my water cool for over 4 hours on a recent hike - it even had room for my small essentials

• The Hydro Flask Journey Series hydration pack is designed to hold three liters of water and keep it cold for up to four hours in an insulated reservoir and sleeve. It also has a side pocket for small essentials.
• Starting at $165, the backpack is designed with a modeled mesh back panel aimed to stop body heat transfer but still allow air to flow between your back and the hydration pack.
• I used the 10 L Hydro Pack recently to hike more than seven miles through the Rocky Mountains. My water remained cold the whole day, and the bag felt comfortable enough to carry throughout my hike.

If you're going to be hiking, biking, or just spending more than a couple of hours outdoors, chances are a standard bottle of water isn't going to cut it. As is, it's recommended that you drink at least eight 8-ounce glasses of water a day, so I quickly realized I was going to need a large water bottle when hiking in Colorado.

But in addition to needing something that'd allow me to carry large quantities of water, it had to do so comfortably. Most of the Insider Picks team owns and loves their Hydro Flask water bottle, cup, or mug, so 95% of my problem was already solved.

The Hydro Flask Journey Series is a line of hydration packs, basically insulated bags of water housed inside backpacks with a straw that goes from the pack and around your shoulders for hands-free drinking.

The bags comes in two different sizes - 10 L ($165) or 20 L ($200) - depending on how much extra room you need for other items like keys, a wallet, suntan lotion, and more. Each pack comes in two sizes - S/M or M/L - depending on your height and hip width. Regardless of the backpack size, the inner bag can hold up to 3 L of water. I'm only 5 feet 4 inches tall, so I went with the 10 L, S/M bag.

The exterior of the bag, seams, and zippers are taped and coated for a waterproof design. It also has two exterior zippered pockets - an easy access side pocket and a front pocket on the 10 L pack, and a top pocket on the 20 L pack. I used the side pocket to hold my phone and keys and the front pocket to hold my Nature Valley granola bar, inhaler, and wallet. That's pretty much all I could fit - minus the water, of course.

If you tend to overpack or need more supplies than what I'd brought, the 20 L version is definitely the way to go.

The interior main pocket contains a neoprene sleeve with reflective lining to keep the heat out and the cold in for approximately four hours. This is where you'll put the 3 L reservoir, which is also insulated and sweat-proof, so your water stays inside the reservoir and not all over your backpack.

There's also an additional elastic pocket in the main compartment for other items to be stored securely.

The reservoir itself is very thick and durable with a fold-down seal. I threw maybe a handful of ice cubes into the reservoir and filled it with cool tap water. For testing purposes, I flipped the bag upside down, applied light pressure, and tossed it around a bit with not so much as a drop of water escaping.

When slipping the filled reservoir into the sleeve, I did have to do a little shoving and jiggling to get it in. Once in though, it was easy to hook up the connective system and bite valve to the reservoir and through the access hole so I'd be able to drink it hands-free while hiking. Additionally, one of the shoulder straps comes with a removable magnetic clip to keep the connective system out of your face while you hike and explore.

Once I slipped the shoulder straps on and buckled the supportive chest and waist straps, I was all set to hit the trails at Rocky Mountain National Park.

The first thing I noticed when putting the full hydration pack on was how comfortable the weight of the water and the pack felt on my shoulders and back. I was a little concerned with how heavy it might feel after several miles in the high altitude, but I was able to go more than seven miles without ever once feeling uncomfortable. Wish I could say the same about my shins and ankles.

Another really great feature, and probably my favorite part of the pack, is the articulated, or molded, back panel which serves two purposes. First, it's designed to keep the pack and reservoir off of your back to avoid body heat transfer to the cold water. Second, it allows air to flow between your back and the pack which I was very grateful for since there wasn't a single cloud in the sky on the day of my hike.

Seven miles and definitely more than four hours later, my water was still refreshingly cool, my back fairly sweat-free, and my body well-hydrated. Although my ice cubes were completely melted, my water was still pretty cool - it wasn't warm or even barely room temperature despite being on my back and under the sun for hours. The additional specs like the back panel and reflective sleeve lining definitely helped keep my water cool throughout my entire hike.

As far as the price goes, yes, it is expensive for what's essentially a water bag.

The 10 L bag costs $165 while the 20 L style is $200. Those prices might seem a little steep upfront, but having a backpack designed to hold a lot of water - and keep it cool too - definitely goes a long way in terms of functionality, comfort, and hydration when you're being active for hours. I didn't have to stop and grab my water bottle from my backpack every so often, and my water was still cool at the end of my hike, so the benefits were definitely worth it for me.

I'm glad that I had my Hydro Flask Journey Series Hydration Pack on my hike through the Rocky Mountains. I was never concerned about running out of water or drinking nasty sun-boiled water, and I spent my time soaking up the beautiful mountains instead of fidgeting with a regular backpack and water bottle.

If you're just in the market for a reservoir and not backpack, CamelBak makes affordable and durable reservoirs ranging from 1.5 L ($30) to 3 L ($35). You can probably put these inside a standard bookbag and jerry-rig it somehow, but I'd imagine having a backpack specifically for your reservoir is more convenient.

Buy the 10-L Journey Series hydration pack from Hydro Flask for $165

Buy the 20-L Journey Series hydration pack from Hydro Flask for $200

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