It is a part of, and lies off the western shore of mainland Skagit County.
The island has a land area of 4.109 km2 (1.586 sq mi) and is home to only a few private residents.
It honors Arthur Sinclair, who was captain of General Pike during a naval battle of the War of 1812.
[1] Settlers in the 1890s referred to the island as Cottonwood Island, after the lumber harvested there for making barrel heads and staves.
This Skagit County, Washington state location article is a stub.