Yup, recreation .gov, but only in C loop, A & B loops are first come first served.
I just stayed this weekend and had reserved my site about 5 months in advance. We checked last week if we could get another campsite and they were 100% booked so i would definitely plan ahead. You can book on the website recreation.gov
I believe camping is on a first come first served basis. Check the forestry dept website to be sure.
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