You can reserve via phone or online. You can also pay when you get there, they have self-pay envelopes. Only risk of not reserving a spot is site availability. Enjoy your visit it is a cool place!

Jayelle, I believe Arizona State Parks has moved to advance reservations only and if so you would pay for your site when you reserve it.

I reserved via azparks.com I believe there are staff present 7 days a week so i think it safe to pay on arrival.