Sinclair Island is an island in the U.S. state of Washington. 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.
The name Sinclair was given by Charles Wilkes during the Wilkes Expedition of 1838–1842. It honors Arthur Sinclair, who was captain of General Pike during a naval battle of the War of 1812.
Settlers in the 1890s referred to the island as Cottonwood Island, after the lumber harvested there for making barrel heads and staves.
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References Sinclair Island: Blocks 1029 thru 1036, Census Tract 9501, Skagit County, Washington United States Census Bureau