You're halfway through a long ride on a hot summer day. Dappled light breaks through the canopy overhead, and as you round a corner, the sound of a brook comes into earshot and you decide to stop and cool off. You bend to the water and splash some on your face. It's cool and refreshing. Turning back to your bike, you notice that you're getting low on water. With only a quarter bottle left, you're going to run out well before returning to the trailhead. You look back at the brook. And then back at your bottle. And then back at the brook. The water is clear, and you're out in the middle of the woods. It's probably fine, you say under your breath. Indeed, it probably is.

But is it? Even seemingly pristine backcountry water sources can contain protozoa—especially cryptosporidium and and giardia—and bacteria, such as salmonella and E .coli. All of these delightful characters are spread through the fecal matter of humans and, significantly, animals. The closer your water source is to human populations or livestock, the higher the odds are that it's contaminated. Even deep in the wilderness, there's a small but real potential that you could pick up an unwelcome stowaway.

“Just carry more water,” you say. Sure, you could, but if you know there are water sources along the trail, why not lighten your load by a few pounds and just bring a filter instead? As they say, “Give a man a fish and you feed him for a day, give a man a water filter and he won’t have any fish, but he will have clean water.”

 

Filtration and Purification 101

Back to protozoa and bacteria. They can be removed with what's called a mechanical pump filter. These devices pull untreated water through an element—usually carbon, ceramic, or fiber—that catches sediment and some microbes.

Wait, only "some?" There's a third class of invisible creepy crawler that lurks in dubious water sources: viruses. Because they’re so tiny, only a few larger and more expensive mechanical filters are capable of filtering them, and they aren’t ones I'd consider carrying on a day ride.

The good news is that viruses don't generally jump from one species to another, so if you're in an area with very little human activity, the water probably won't contain any viruses that could infect you. If, on the other hand, you're taking water from a lake that people swim in, or from a source near campsites, you should consider purifying the water. Unless you're carrying a big, honkin' $400 filter that also purifies, this will require the use of either a UV device or chemical tablets.

 

Our Pick for Filtration: MSR TrailShot — $50

This fist-sized filter will fit in a bib pocket, fanny pack or backpack, but portability is only part of why it accompanied me on the majority of my rides over the past season. The TrailShot's hose makes it easier to draw from the water source, and allows you to really pick and choose where you're filtering from. The hose also makes it possible to pull water from some very shallow spots—puddles, even. Once the hose is in the water, squeeze the bulb to draw water through the hollow-fiber filter element and out the spout. Filling a water bottle takes about 30 seconds, and you can also drink straight from the filter.

Best Water Filters for Mountain Biking
Best Water Filters for Mountain Biking

MSR says the cartridge is good for 2000 liters, which means that you could refill a 16-ounce bottle over 4,000 times. Needless to say, I can't confirm MSR's claims about the filter's longevity, but it's still working for me after a season. The flow rate will slow with use, especially if you're pulling in a lot of sediment, but cleaning the filter is simple enough.

 

Filtration Alternatives

I also experimented with the LifeStraw Flex ($35). It’s an excellent option for use with a pack, since it can be installed inline in the hydration hose. With this setup, you can fill your hydration pack directly, and then the water will pull through the filter as you drink.

Best Water Filters for Mountain Biking

The LifeStraw comes with a soft bottle that it screws into, but it obviously doesn't fit in a cage. You could fill the included bottle and then squeeze the water out of that bottle and into your own, but the MSR is better designed for that and just about every use other than as an inline filter with a hydration pack.

 

Our Pick for Purification: Steripen Adventurer — $100

UV purification devices like the two I tried out from Steripen emit a specific wavelength of UV light that kicks microorganisms right where their DNA strands code for reproduction. So even if you consume them, the little buggers can't infect you because they can't reproduce. If your water is clear, you might be okay to skip straight to purification. But if it's turbid or murky, you're best off filtering first, since sediment can shield microorganisms from the UV light.

Best Water Filters for Mountain Biking
Best Water Filters for Mountain Biking
Best Water Filters for Mountain Biking

Once you're ready to purify, activate the Steripen, dip it into the water, and stir until it tells you to stop (about a minute). Just be careful to clean the outside of the bottle, and note that any droplets inside the bottle but above the water line won't be purified. For this reason, I like to shake the bottle after the first treatment and then hit it again with the UV light.

Purification Alternatives

Water purification tablets have the advantage of cost and size, but that's about it. They use chemicals—usually chlorine, chlorine dioxide (used in municipal drinking water systems), sodium or iodine—to neutralize bacteria and viruses. Most won't kill cryptosporidium, but a few will. Regardless, they all require at least 30 minutes to finish the job, and the murkier and colder the water, the longer they take. Plus, they usually produce a funky taste and odor.

The Ideal Setup

You should never ride off into the wilderness without enough water, just banking on there being sources along the way. But if you know that there's water where you're headed, bringing a filter is a smart way to lessen your load and ensure that you have enough to drink. Here are a few scenarios to help you decide on the best filter for your ride:

Your water source is…
A beautiful, clear lake. So beautiful that people often camp on its shores.
If the water is clear and still, you probably don't need a filter. But you should purify the water to guard against microorganisms from unsanitary humans.

A forest stream that runs downhill. There are no livestock near the source, and there’s no human activity or camping in the area.
You're probably fine to simply filter the water with something like the MSR TrailShot.

A valley river in an area with both livestock and humans.
I'd bring both types of filters. Especially after rainfall, valley rivers tend to be very turbid, and if there are animals and potentially even humans doing their business on the mountains that feed into the river, you'll want to filter out the sediment so that the UV purifier can work its magic.