Idaho Code § 31-128
Jefferson county is described as follows: beginning at a point where the township line between range thirty-one (31) and range thirty-two (32) east, intersects the township line between townships three (3) and four (4) north;
Southern boundary. Thence east along said township line between townships three (3) and four (4) north, to the southeast corner of section thirty-three (33), township four (4) north, range forty-one (41) east;
Eastern boundary. Thence north on the section line between sections thirty-three (33) and thirty-four (34), township and range aforesaid, to the northeast corner of section twenty-eight (28), said township and range; thence west on the section line between sections twenty-one (21) and twenty-eight (28), said township and range, to where the same intersects with the center of the main channel of the South Fork of Snake river; thence down the main channel of said South Fork of Snake river to the conjunction of said stream with the North Fork of Snake river; thence down the center of the main channel of Snake river to a point where the same intersects the section line between sections twenty-one (21) and twenty-two (22), township five (5) north, range thirty-eight (38) east; thence north on said section line between said sections twenty-one (21) and twenty-two (22), township and range last aforesaid, to the northeast corner of section four (4), township seven (7) north, range thirty-eight (38) east; thence west nine (9) miles, to the southeast corner of township eight (8) north, range thirty-six (36) east; thence north six (6) miles, to the northeast corner of said township eight (8) north, range thirty-six (36) east;
Northern boundary. Thence west on the township line between townships eight (8) and nine (9) north, thirty (30) miles, to the northeast corner of township eight (8) north, range thirty-one (31) east;
Western boundary. Thence south (1913, ch. 25, section 2, p. 95) along the township line between range thirty-one (31) and range thirty-two (32) east (1917, ch. 98, section 2, p. 346), to the place of beginning.
County seat-Rigby.
Idaho Code § 31-128