Idaho Code § 31-121
Custer county is described as follows: beginning at the confluence of the Pahsimeroi river with the Salmon river thence up the Pahsimeroi river to the mouth of the Big creek; thence up Big creek, and on the line from the head thereof, with the general course of said creek to the summit of the divide between the waters of the Pahsimeroi and Lemhi rivers; thence southeasterly on the summit of said divide to a point west from the headwaters of said Little Lost river; thence east to the headwaters of said Little Lost river, thence down Little Lost river to the township line between townships ten (10) north and eleven (11) north; thence west along said township line to the corner common to townships twenty-five and twenty-six (25 and 26) east and townships ten (10) and eleven (11) north; thence south along the line between townships twenty-five (25) and twenty-six (26) east to the corner common to townships eight (8) and nine (9) north; thence west along the line between townships eight (8) and nine (9) north to the northwest corner of section two (2), township eight (8) north, range twenty-five (25) east B.M.; thence south along the west section line of section two (2) and eleven (11) to the northwest corner of section fourteen (14), township eight (8) north, range twenty-five (25) east B.M. to a point which is on the summit between the divide of Pass creek and Little Lost river; thence down said Pass creek to Big Lost river; thence along Big Lost river to the mouth of Antelope creek; thence up Antelope creek to the divide which separates its waters from those of Little Wood river; thence westerly along and upon the summit of the range of mountains dividing the headwaters of the East Fork of the Salmon river from the waters of the Little and Big Wood rivers, and continuing westerly on said divide between the East Fork of the Salmon and Wood rivers to the intersection of the longitude line of longitude one hundred fourteen (114) degrees, forty (40) minutes west from Greenwich, thence north on said longitude line to a point on said divide due east of the northeast corner of section twenty-four (24), township seven (7) north, range fourteen (14) east; thence due west to the northeast corner of section twenty-four (24), township seven (7) north, range fourteen (14) east; thence west along and upon the section lines to the corner of sections fifteen (15), sixteen (16), twenty-one (21) and twenty-two (22), township seven (7) north, range fourteen (14) east; thence north along and upon the section lines to the corner of sections three (3) and four (4) and thirty-three (33) and thirty-four (34), townships seven (7) and eight (8) north, range fourteen (14) east; thence north along and upon the section lines to the one-quarter (1/4) section corner between sections twenty-seven (27) and twenty-eight (28), township eight (8) north, range fourteen (14) east; thence west along and upon the one-quarter (1/4) section lines to the west one-quarter (1/4) section corner of section thirty (30), township eight (8) north, range fourteen (14) east; thence southwesterly along and upon the summit of the mountains dividing the waters of Yellow Belly lake and Pettit lake to the summit of the Sawtooth mountains; thence northerly along the summit of the Sawtooth mountains to the divide which separates the waters flowing into the South Payette river and Bear Valley creek from those flowing into the main Salmon river and Cape Horn creek; thence along said divide to the Middle Fork of the Salmon river; thence down the Middle Fork of the Salmon river to the mouth of Loon Creek; thence up Loon creek to the mouth of Warm Springs creek; (thence up Warm Springs creek); thence up Warm Springs creek and to the divide which separates the waters of Yankee Fork on the south and Loon and Deep creeks on the North, and following said divide in an easterly direction around the head of Panther creek, to the divide between Hat creek and Ellis creek; thence on the divide between Hat and Ellis creeks in an easterly direction to the Salmon river; thence up the main channel of said Salmon river, to the place of beginning.
County seat-Challis.
Idaho Code § 31-121