If you hang out around old school backpackers and river runners, you might here stories of drinking water straight from the streams, however, drinking from most rivers, creeks, and streams puts you at risk for getting Giardia. There are several ways to avoid getting Giardia and to make your drinking water fresh and tasty including boiling, iodine or chlorine tablets, and filters.
Boiling Water: Bring the water to a rolling boil for one minute, then let cool and you can drink it. If you are going on a longer expedition, boiling water every day can become tedious and use up all your fuel. But it's a great backup plan.
Iodine: many people don't like the taste of iodine, and some even argue that cryptosporidium can still grow in iodinized water, however it's an easy way to treat water and very inexpensive. Iodine tablets are easy to come by at most outdoor stores and the directions for using them are usually on the bottle.
Aqua Mira: the latest and greatest (but also tried and true) method for purifying water in the outdoors is Aqua Mira droplets. You mix a one-to-one ratio of droplets from the two bottles to create chlorine dioxide and it kills virtually everything in your water including Giardia and Cryptosporidium. The best part about using Aqua Mira to purify water is that it doesn't taste like anything.
Bleach: This is not a recommended method simply because it makes the water taste like bleach, and it is easy in the backcountry to over-bleach your water, however it is good to know that in a pinch, it will get the job done. The recommended amount is 8 drops per gallon, although this makes the water taste awfully bleachy. If you are planning on purifying your water using household bleach, make sure to carry a water dropper with you as well.
Filter: One of the most used methods of purifying water is to filter your water is through a water filter pump. We are not talking Britta filter here. We are talking about something more akin to a Katadyn or Pur filter made with glass, ceramic, and charcoal. Water filters come in all shapes and sizes, with the largest ones filtering water for big groups on expedition style trips (think Grand Canyon) and the smallest ones coming as a package with your water bottle.
Sometimes finding the right filter can be incredibly confusing, mostly because there are filters that take out chemicals from tap water, and filters that actually remove amoebas and bacteria in the outdoors. Many people prefer water filters because they leave your water tasteless, rather than tasting like iodine or chlorine. The downsides to water filters is that they are bulkier and can be heavy. Additionally, the filters can get clogged. If you are filtering water that is sandy or very dirty, put it in a bucket and let the water settle before trying to filter it. Once the dirt has settled to the bottom, filter the water from the top of the bucket.