Of course! Pop your reservoir in the freezer overnight so you can head out knowing you’ll have cold water for a little while. Your hydration pack naturally insulates the bladder, so adding ice will help keep the water cool even if it’s hot outside. Keep in mind that condensation may occur and cause some dampness inside the pack. Avoid putting anything that isn’t waterproof close to the bladder. Freezing the reservoir after cleaning is a good way to prevent mold growth, too.