This article is from the Hiking/Backpacking With Canines, by Terri Watson with numerous contributions by others.
Clean drinking water is a must for both you and your dog. Although
natural water sources may be plentiful on your hike, the water my be
contaminated with giardia (a protozoan parasite), or harmful bacteria
or chemicals. In areas where giardia is a problem you should not allow
your dog to drink from steams or lakes on your hike. Always make sure
that you carry enough water for your hike. See the section on weather
below for additional information.