The Island Unit is located between Steamboat and Freshwater Sloughs in the Skagit River’s South Fork in Skagit County.
https://github.com/davepinch/pinchy.cc/blob/master/content/topics/places/sloughs/steamboat-slough-skagit-river/steamboat-slough-skagit-river.md
Boundary Bay (Wikipedia) Boundary Bay is a shallow bay situated on the Pacific coast of North America on the Canada–United States border between the Canadian province of British Columbia and the U.S. state of Washington.