Scouts on outings are always asking me two questions: how much further do we have to hike, and whether or not it's safe to drink the water. Unlike the first question, the answer to the water issue isn't always a simple one, but things have changed since my scouting days (mostly due to an increased human population in the backcountry) when we always drank directly out of streams and lakes and never seemed to get sick. There are times when I will still drink untreated water straight from the earth, and it is one of the world's great simple pleasures, but those times are becoming fewer and fewer.
Some of the things you need to consider are how much distance is between you and the actual source of the water (usually melting snow), what time of year it is (later in the year when the water is warmer and bodies of water are smaller is worse), what magnitude of human and/or animal traffic is present around your water source (or between you and the source), whether the water is moving or not, and the look and smell of the source. As with most backcountry decisions, the final responsibility is yours alone – you must decide whether or not to treat your water, and live with the consequences.
To be really safe, you should always filter your water or treat it by boiling or chemicals before consuming it.
If and when you decide to treat your water, you have a number of choices. You can boil your water (if you are cooking with it anyway, there is no need to filter) or treat it with iodine (you must let the water stand for a while and there is some residual iodine taste). For a quicker and tastier final product, there are water filters, which are designed to cleanse relatively clean water sources of larger bacteria (normally those over 2 or 3 microns), particle matter and protozoans (such as Giardia and Cryptosporidia). These units are normally sufficient for backpacking and camping purposes away from population centers and at altitude. Water purifiers take the cleansing process a step further, and remove or kill viral and bacterial contaminants as well by either mechanical or chemical means (or both), usually involving iodine or chlorine. Purifiers are desirable when camping in certain other parts of the world or when the probability of your water's contacting human or animal contaminants is high.
In addition to the choice between filters and purifiers, you need to consider what quantity of water you will probably be processing. Some very compact units, such as the Katadyn Minifilter, the MSR MiniWorks Filter, and the Sweetwater Walkabout, are suitable for solo travelers or possibly two hikers, while other models are better suited to patrol use or even treating enough water for a large scale climbing expedition.
For patrol use on backpacking trips, slightly larger units such as the MSR Waterworks II, the Pur Guide, or the Sweetwater Guardian are advisable – they are somewhat heavier, but capable of filtering larger quantities of water in less time. It's a good idea to bring a collapsible water container holding several gallons and filter it all at one time (or bring it to your campsite and filter as needed) so that you limit the number of trips you take to your water source.
MSR has recently introduced a totally new and different type of water treatment, dubbed MIOX, which works by producing a dose of "mixed oxidants" with a pen-shaped device (it requires camera batteries and salt to operate) which are then added to a volume of water and let stand.
(from the MSR website):
Developed in conjunction with the U.S. military, the MIOX® Purifier offers unrivaled ease of use and reliability for purifying large volumes of water, making it ideal for outdoor recreation, travel, and disaster-preparedness. It works by creating a powerful dose of mixed oxidants (MIOX), which is then added to untreated water, inactivating all viruses, bacteria, Giardia, and Cryptosporidium (which even iodine doesn’t kill). And since the MIOX Purifier needs only common camera batteries and salt to operate, it’s maintenance-free, delivering more water, more easily than any purifier ever before.
Unless you plan on camping in Third World countries near animal or human populations or need to process water in the event of a natural disaster, water purifiers are probably not necessary for scout outings in the Northwest, but it doesn't hurt to have the capability of eliminating viruses and bacteria if you have the extra money and don't mind carrying the weight. Sweetwater offers a chlorine-based additive called ViralStop that comes in a small bottle and is added after filtering.
In the interest of preserving fresh water for all users, scouts should always limit travel and play in or near water sources. Pay strict attention to the Leave No Trace guidelines on distances to water sources when cooking and relieving yourself, and never use soap (biodegradable or not!) to bathe or clean dishes with when using a lake or stream.
REI's flagship store on Yale Avenue is a good place to go to compare filters and purifiers – many are set up next to a small pool of water and you can try them out and see which works best for you. Typically, the filter cartridge is the most expensive part of the unit, so try to keep it clean and always use the silt-screen that comes with the filter to keep dirt out of the mechanism. Running silt or mud through a filter will usually damage the filter cartridge or pump permanently.
When you return home from an outing, be sure to clean, rinse, and air dry your filter before putting it back with your camp gear – if you don't, some pretty foul organisms can take up residence inside your filter and make whatever comes out of it on your next trip undrinkable!
If you plan on doing much backpacking as a scout or as a family, a good water filter or purifier is almost essential; ideally each patrol should have their own filter on an outing. Get one you like, and make sure you understand how to use and maintain it. Now get out there and hydrate!