It depends how many visitors that day.
It depends how many people go that day, I've been there around 5 or 6 and no one was there at those times I had reservations every time also.
It honestly doesn't matter if you have a reservation or not. If there are a lot of people showing up to the park you're going to wait, possibly a few hours. It's the worst Park to check into.
I've checked im around 4 or 5 and there was mo line but it took about 45 minutes to check in and get our reserved campsites.
The real busy days are Friday, Saturday and Sunday. Usually 2 hour wait. Checking in on Tuesday, Wednesday or Thursday not so much
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