I agree with Muirenn. Anything waterproof in the mid-70s is going to feel like a sauna. Even water resistant--in my experience--can be uncomfortably warm. I'd just as soon get wet than overheat. But, as others have said, make sure to keep to anything that needs to stay dry in a ziplock bag(s). You could put a plastic bag around your camelbak or just accept that it's going to get wet. I assume that this is a relatively flat ride given where it is, but if it's not, be extra careful descending. Keep a close check on your speed.