I always make my reservations thru recreation.gov you can only book 6 months in advance.
Like Emerald said, best to go through recreation.gov. It's a great campground. We were there in July. Watch for cows when going around blind turns though....
I checked the government website when we were in the area and it said they had spots, so we just showed up. This was in May, the very beginning of the season.
This place books a year forward got us
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