In cold-weather seasons, be particularly careful to avoid hypothermia, the lowering of the body's core temperature. Hypothermia kills one or two people a year in the park, even in the summer, because they didn't have warm clothing handy after they went wading in a cold stream and came out into a sudden rainstorm. Don't be surprised if you feel a little short-winded while you're in the park. You're probably not used to the higher elevations. If you're going to be in the woods for an extended time, take plenty of water with you and drink it.
Dehydration is a by-product of oxygen deprivation at high altitudes.