Idaho Code § 31-103
Ada county is described as follows: beginning at 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, crosses said Boise river;
Northern boundary. Thence down the center of the channel of the Boise river to a point opposite the mouth of More's creek. Thence in a straight line north forty-four (44) degrees and thirty-eight (38) minutes west until the said line intersects the north line of township five (5) north (12 Ter. Sess. 67); thence west to the northwest corner of township five (5) north, range one (1) west;
Western boundary. Thence south to the northwest corner of township three (3) north, range one (1) west; thence east to the northwest corner of section four (4), township three (3) north, range one (1) west; thence south to the southeast corner of section thirty-two (32), township two (2) north, range one (1) west; thence west to the northwest corner of township one (1) north, range one (1) west; thence south to the point in the middle of the channel of Snake river, where the line between township one (1) south, range one (1) west, and township one (1) south [, range two (2) west, crosses the said river];
Southern boundary. Thence southeasterly up the center of the channel of the said 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 crosses the said Snake river;
Eastern boundary. Beginning at a point in the center of the channel of the Snake river 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; thence extending 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, the point of beginning.
County seat-Boise City.
Idaho Code § 31-103