Colo. Rev. Stat. § 30-5-163
The county of San Juan is hereby created and established, with the legal capacity and functions of the other counties of the state, and with boundaries as follows:
Commencing at a point on the western boundary of Hinsdale county, nine miles south of where it intersects the tenth correction line north; running thence due west to the western line of the state; thence north along such line to a point ten miles north of the thirty-eighth parallel of north latitude; thence east to a point on the summit of the main range of mountains, dividing the waters of the Uncompahgre and Animas rivers from the waters of the Lake Fork of the Gunnison; thence in a southerly direction along the summit of said range to a point at the head of the north fork of Pole creek; thence down said creek to the present boundary line between Hinsdale and La Plata counties; thence due south along said line to the place of beginning; and the boundary lines of La Plata and Hinsdale counties are hereby changed to correspond with the provisions of this section.
C.R.S. § 30-5-163