Fort Stevens SP, Oregon

I would get reservations and if that doesnt work, on the Washington Side of the Columbia is Cape Disappointment, also an awesome park

Ft Stevens is full practically year round. Try gwtting a reservation for the dates you want to visit. BTW, there is a KOA right across the street from the enterence if FT Stevens is full.
Reservations northwest handles all OSP reservations.

I'd call the park and ask. ALthough we've been to Ft Stevens many times in Spring or Fall during our travels and never seen it even half full. We never go during summer months though, and don't know when school lets out.

If dates are real important, I'd make the reservation. Otherwise, take a chance. It's a really nice park ... as are all Oregon parks.