From your description, the bird was in Columbia County. The Reeder Road platform is located in Columbia County at least a couple hundred yards north of the County line. The wetlands some distance to the south are in Multnomah County, but most of the viewable ponds and marsh are in Columbia.
As for the bird, it's a fantastic sighting either way. I think you would need to carefully rule out Common Tern, which it sounds like you have, but my guess is a Forster's is probably more likely inland at this time of year anyway. I think 2Y individuals of most medium tern species that retain basic/immature plumage stay on the wintering grounds, but some are seen elsewhere every summer.
Any chance you took any photos?
On Tue, Jun 12, 2018, 1:48 PM Bill Shanahan <iamshanahan...> wrote:
> Reeder road observation platform (not sure where the county line is). 11 > am today. > > First sitting, then flying around and diving, finally disappearing to the > northwest. > > Weird part is it did not appear to be in breeding plumage. Tail, legs, > bill, and facial markings were perfect for non breeding Forster's. Thoughts? > > >