There is a way to carry a lightweight water filter in your pocket. It's called the Aquamira Frontier Filter (available from the company at www.aquamira.com or from other online sources). This little filter is used just like a straw, and it breaks down into two very compact parts so it's easy to carry in a shirt pocket. And it weighs next to nothing. Not only that, but it will screen out giardia and cryptosporidium, the two most prevalent illness-producing waterborne organisms in North America.
The filter is rated for 20 gallons, assuming that you don't suck a lot of silt into it. Cloudy water will quickly clog any kind of filter, so it's important to allow particulates to settle to the bottom and then draw your drinking water from the clear top layer. You can help clear the water by spreading a t-shirt on the surface and allowing the fabric to act as a pre-filter by catching silt and allowing clear water to pass through. Then you would draw your drink from the clear water above the shirt.
Use this filter just like a straw, to suck water directly from any body of water. Even a small puddle can provide lifesaving liquid.
Aquamira also makes larger filtration systems, but this one is my favorite as a grab-and-go unit for short trips into the mountains or desert. With a 20-gallon capacity, it would provide almost 3 weeks worth of survival water, if I were drinking a gallon per day.
For about $12, it's hard to beat this essential piece of survival equipment.