Although finding water on the Appalachian Trail generally is not a problem, water purity is the concern. Most shelters are situated in close proximity to a spring, and many hikers rely on the fact that if the water runs directly from the ground, it is usually safe to consume.
Reliable and natural water sources are listed in guidebooks like Appalachian Trail Data Book and Thru-hikers' Guide to the Appalachian Trail, though treating or boiling all water regardless of the source is by far the safest option.
The protozoan Giardia lamblia is the most abundant and debilitating parasite in the waters of the Appalachians and thus, the biggest worry for hikers. Symptoms of infection include diarrhea, abdominal cramps, fatigue and vomitting. These usually appear 6-20 days after ingestion, last 10-14 days, and are treated by prescription drugs.
Hikers generally treat their water in one of three ways, boiling, water filters/purifiers, or by some kind of chemical or tablet. Boiling is one of the most reliable ways to protect yourself from infection, though it is time consuming and relies on fire or fuel. If you choose to boil your water, you should do so for at least one minute and between three to five in high altitudes. Options for chemical and tablet treatment include chlorine dioxide and iodine. These, however, are not reliable for killing Giardia and Cryptospridium. The effectiveness of filters and purifiers varies greatly depending on the make and manufacturer. Options for these include bottle filters, gravity feed filters, and pump filters. The pump filters are generally faster and more effective but also require more maintenance. If you choose a filter or purifier, be sure to do your homework and to purchase one from a reputable dealer. It is also important to know how long your filter will function before it needs replacement. Filters generally weigh between 1 and 1.5 pounds.
Most thru-hikers use a collapsible 2.5 gallon water bag for supper, cleaning, breakfast and filling canteens. Filling the water bag will be one of the first tasks completed upon arriving at a shelter or campsite. After emptied, the bag collapses nicely and adds little weight to packs.
Awol's latest AT Guide or the ATC's Thru-Hiker's Companion can't help you quench your thirst on the Trail when water conditions are this dry.