The San Juan Islands are a unique watershed, with no major rivers and a few streams fed only by groundwater and precipitation. Second-growth conifer forests, hardwood forests, shrub plants, prairie, and rocky beaches all occur on the islands, but forest is dominant over almost 70 percent of the county. The islands get less rainfall than the rest of Puget Sound, a result of the rain shadow from the Olympic Mountains and Vancouver Island. At the south end of Lopez Island, precipitation is around 19 inches annually, rising to 30 inches at the northern tip of Orcas Island. Because the only source of freshwater is rainfall, water availability is a concern.
The islands support over 291 species of birds, including the largest bald eagle population in the lower United States. Fresh- and salt-water wetlands provide important habitat for a number of species, as does the upland prairie found on several of the islands.
- Counties: San Juan
- National Estuary Programs: Puget Sound
- Other Watersheds Upstream: None
- Other Watersheds Downstream: None