They have a number of first-come first-serve sites. In fact, some of them are better than the sites available for reservation.

Sunday night thru Wed night you will NOT need a reservation. Thursday night is iffy. Far night and Sat night will almost surely require a reservation.

We stayed there a couple of times without reservations - but not peak season.

We made reservations when we stayed - not sure if they have any first-come first-serve spots or not…