Latest we were allowed to check in before they went checking sites was 7:30. Yes, I recommend doing a reservation. There are some sites, first come, first serve, but definitely recommend reservations.
You can get to your site any time after 4pm and be gone before 2pm the next day. You check in with the camp host if they are at thier site. If not just proceed to your campsite. Reservations are recommended for most sites, some can not be reserved. They dont close a gate at arrowhead campground. They are open 24/7. There is no time limit to check in.
They close the gate at 10 pm. They do have some walk-up sites but during the spring and summer I would strongly encourage you to make reservations.
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