Both Colter Bay and Gros Ventre will assign you a site when you check in, but if you find one you prefer after driving around, you can go back and they will usually allow you to change spots. We have a number of sites we like at Colter Bay and usually ask if they are available before we go in. If so, we pay for the site and go park. If they have to assign us a site, we will go look it over and, if we find something, empty, that we like more, we'll just go back to the station and ask to be assigned that spot. It's usually pretty easy to do. Colter Bay is our favorite spot as we go up to fish on Jackson Lake as it sits very close to the marina where we launch our boat. It's also closer, by about 20 miles, to Yelowstone National Park compared to Gros Ventre.