Quamash Prarie is a natural area owned by Metro. Try a Google search to see some of the rare wildlife here. Sometimes you can hear the coyotes and I've seen bobcats tracks here too.
They don't want caches on site, hence the roadside hide. You're looking for a black camo match tube. Best parking is to approach from Scholls Ferry and park on the wide shoulder rather than a little later at the gate.
Congrats to Scott 45N for the FTF.