Alex, are you riding at a different altitude than you live? If it is the same, it's really only the humidity that you need to worry about in addition to the heat. If you're riding at higher altitude than you live, your respiratory rate may increase and you'll loose water through more rapid breathing.

MP, correct me if I'm wrong.

If you live in SLC at 4,400-5,400 ft (depending on your location) and you're riding in the same general area/altitude, you should only have the humidity and heat to worry about. If you live at sea level and go ride at 5,000 ft, then your breathing will quicken and you'll loose water more quickly.

I'm in agreement with everyone on the over-hydration.