Canyon County is the second-most populous county in Idaho. The county seat is Caldwell and its largest city is Nampa. Canyon County is part of the Boise Metropolitan Statistical Area. The county originally contained all of Canyon and Payette counties and part of Gem; Gem County formed in 1915 and Payette County in 1917. Some sources attribute the name to the canyon of the Boise River near Caldwell, while western writers John Rees and Vardis Fisher believed it was named for the Snake River canyon, which forms a natural boundary with Owyhee County to the south and west.

Ada County has a total area of 1,060 square miles, of which 1,053 square miles is land and 7.9 square miles is water. The Boise River flows through the northern portion of the county, and the northwest border is bounded by the foothills of the Boise Range mountains, the summits are in adjacent Boise County. The southwestern border of the county is bounded by the Snake River.,/p>

Named for the state nickname, "Gem State," the county was established on March 15, 1915, partitioned from Canyon County and Boise County. Fur trappers were in the area as early as 1818, and Alexander Ross explored Squaw Creek in 1824. Prospectors and miners moved through the county in 1862 en route to the gold rush in the Boise Basin around Idaho City, and by the next year irrigation began along the Payette River.

Owyhee County is in the southwestern corner of Idaho. The county seat is Murphy and its largest city is Homedale. It is the second-largest county in Idaho, behind Idaho County. Owyhee County is part of the Boise metropolitan area and contains slightly more than half of the Duck Valley Indian Reservation, which extends over the Nevada border, into Elko County. The majority of the federally recognized Shoshone-Paiute Tribe that is associated with this reservation lives on the Nevada side; its tribal center is in Owyhee, Nevada.

Payette County, Idaho is part of the Ontario micropolitan area. The county was established in 1917, partitioned from Canyon County. It was named after the Payette River, which was named after French-Canadian François Payette. Payette County is one of the few counties in Idaho to be the home to the endangered Idaho ground squirrel.