San Juan, an island of Washington territory, in Washington sound, between the gulf of Georgia on the north, the strait of Fuca on the south, Rosario strait on the east, and the canal de Haro on the west, about lat. 48° 30' N., lon. 123° W.; length 15 m., greatest breadth 7 m.; area, about 60 sq. m.; pop. in 1870, 376, including the American garrison of 98 men, but exclusive of the British garrison. The N. part is mountainous and heavily timbered; the S. part has many beautiful and fertile prairies, and excellent pasturage. Coal and limestone are found. The adjacent waters abound in cod, halibut, salmon, and other fish. This and several smaller islands were included in Whatcom co. till 1873, when they were formed into the county of San Juan. The largest of the other islands are Orcas, about 60 sq. m., and Lopez, about 30 sq. m. The rest have an aggregate area of about 50 sq. m., the principal being Blakely, Decatur, Shaw, Waldron, Henry, Spieden, Stuart, and Sucia. Total area of the county, about 200 sq. m.

The population of Orcas island in 1870 was 108; of Lopez, 48; of the others, except San Juan, 22; and of the entire group, 554, including 72 Indians; white population in 1874, 545. - San Juan derives its chief importance from the dispute respecting its possession between Great Britain and the United States. The treaty of June 15, 1846, for the settlement of the Oregon boundary, fixed upon the 49th parallel as the line to "the middle of the channel which separates the continent from Vancouver island, and thence southerly through the middle of said channel and of Fuca's straits to the Pacific ocean." Subsequently Great Britain claimed that Rosario strait was the channel intended, while the United States insisted upon the canal de Haro, leaving Washington sound with its numerous islands in dispute. In November, 1859, an arrangement was entered into between the two governments for a temporary joint military occupation, in pursuance of which a British garrison was established in the N. part and an American garrison in the S. By article 34 of the treaty of Washington, May 8, 1871, the question in dispute was referred to the arbitration of the emperor of Germany, who in October, 1872, decided in favor of the United States; and in the following month the British garrison was withdrawn. - See vol. v. of "Papers relating to the Treaty of Washington," published by the department of state (1872).