Idaho Const. art. XVII, § 1

The name of this state is Idaho, and its boundaries are as follows: Beginning at a point in the middle channel of the Snake river where the northern boundary of Oregon intersects the same; then follow down the channel of Snake river to a point opposite the mouth of the Kooskooskia or Clearwater river; thence due north to the forty-ninth parallel of latitude; thence east along that parallel to the thirty-ninth degree of longitude west of Washington; thence south along that degree of longitude to the crest of the Bitter Root mountains; thence southward along the crest of the Bitter Root mountains till its intersection with the Rocky mountains; thence southward along the crest of the Rocky mountains to the thirty-fourth degree of longitude west of Washington; thence south along that degree of longitude to the forty-second degree of north latitude; thence west along that parallel to the eastern boundary of the state of Oregon; thence north along that boundary to the place of beginning.