There is a Park office just past the gate entrance on the left. I don't know if you need to inquire with them. There's usually an employee inside there on weekdays when I go there.
You just got to get there early and grab a pavilion. It is free but 1st come 1st served. You don't have to contact anyone. You have to clean up afterwards. It really a nice park. Have fun and good luck
Both answers are correct, I'd call if you need more then 1 pavilion.
I guess you would have to contact the park office
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