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The area that now comprises Yakima County was part of the Oregon Country at the start of the nineteenth century, inhabited both by fur prospectors from Canada, and Americans seeking land for agricultural and mineral-extraction opportunities. Unable to resolve which country should control this vast area, the Treaty of 1818 provided for joint control. By 1843, the Provisional Government of Oregon had been established, although at first there were questions as to its authority and extent. During its existence, that provisional government formed the area north of the Columbia River first into the Washington Territory, and then (December 19, 1845) into two vast counties: Clark and Lewis.
The Washington Territory was formed as a separate governing entity in 1853. In 1854, that legislature carved several counties out of the two original large counties, including Skamania County. Later in 1854 the new Skamania County was reduced in size by carving out Walla Walla County. This arrangement lasted until January 23, 1863, when Ferguson County was carved out of Walla Walla County. However, the Ferguson County government and boundary was dissolved on January 18, 1865, and its area was assigned to Yakima County three days later.
Since its creation, the Yakima County boundary has been altered two times. In 1883 a portion of its area was carved off in the creation of Kittitas County, and in 1905 a further reduction added to the creation of Benton County.
The Yakama Indian Reservation was created in 1855. However, several tribes felt the agreement creating this reserved area had been completed without sufficient native input, and skirmishes and local war (the Yakima War, which lasted until 1858) meant that the reservation was not fully operational for two decades. The Reservation is the 15th largest reservation in America, covering 1,573 sq mi, and comprising 36% of the county's total area. Its population was 31,799 in 2000, and its largest city is Toppenish.