On April 25, 1854, the Washington Territorial Legislature forms Walla Walla County out of Skamania County. The county seat is set "on the claim of Lloyd Brooks," at what will become the city of Walla Walla. The county encompasses the area north of the Columbia and east of the mouth of the Deschutes River, including parts of the future states of Idaho and Montana -- 450 miles by 200 miles.
By 1861, the population of Walla Walla County was two-thirds larger than was the population west of the mountains. Mining discoveries led to the construction of roads and bridges and new counties were formed out of Walla Walla. Congress organized Idaho Territory in 1863, setting the eastern boundary of Washington Territory.