From the record
“Fort Okanogan was founded in 1811 on the confluence of the Okanogan and Columbia Rivers as a fur trade outpost. Originally built for John Jacob Astor's Pacific Fur Company, it was the first American-owned settlement within Washington state, located in what is now Okanogan County.”
Wikipedia article Fort Okanogan, CC BY-SA 4.0.
One of 16 National Register listings published here for Okanogan County · listed in 1973 · category: Site.
Sources: the Wikipedia article · the case file at the National Archives · the National Register database, National Park Service