Idaho Code § 31-122
Elmore county is described as follows: beginning at a point on the top of the Sawtooth range of mountains, where the counties of Blaine, Boise, Custer and Elmore unite;
Eastern boundary. Thence in a southerly direction along the summit of the Sawtooth mountains to a point where the trail crosses the summit of what is known as the Mattingly Creek divide; thence in a southerly direction along the main divide between the middle fork and the south fork of the Boise river to a point on the divide between Willow creek and Bear creek; thence in a southerly direction on the main divide between Willow creek and Skeleton creek to the center of the channel of the south fork of the Boise river; thence down the channel of said river to the point of intersection with the range line between ranges eleven (11) and twelve (12) east, approximately in township three (3) north; thence south on the range line between ranges eleven (11) and twelve (12) east, to Snake river;
Southern boundary. Thence down the center of the channel of Snake river to a point where the section line between sections thirty-three (33) and thirty-four (34), township five (5) south, range four (4) east, Boise meridian crosses said Snake river;
Western boundary. Thence in a northerly direction along the north and south center line of townships five (5), four (4), three (3), two (2) and one (1) south, range four (4) east, Boise meridian, to the base line and thence in a northerly direction along the north and south center line of townships one (1), two (2) and three (3) north, range four (4) east, Boise meridian, to a point in the center of the channel of the Boise river where the section line between sections fifteen (15) and sixteen (16), township three (3) north, range four (4) east, Boise meridian, crosses said Boise river;
Northern boundary. Thence upon and along the boundary line of the county of Boise to the place of beginning.
County seat-Mountain Home.
Idaho Code § 31-122