Colo. Rev. Stat. § 30-5-143
So much of the county of Gunnison as is included within the following described boundaries shall be set apart and is hereby established as a county, with the legal capacity and functions of other counties of this state, to be called the county of Mesa:
Beginning at the northwest corner of Pitkin county, and running thence south along and with the western line of Pitkin county to the divide between the waters of the Grand river, now the Colorado river, and the north fork of the Gunnison river; thence southwesterly along and with said divide to the southwestern extremity of the Grand Mesa; thence southwesterly to the mouth of the Rio Dominguez; thence due south to the parallel of thirty-eight degrees and thirty minutes of north latitude; thence west to the western boundary line of the state of Colorado; thence north along said boundary line to the northern boundary line of Gunnison county; and thence east along said northern boundary line of Gunnison county to the place of beginning.
C.R.S. § 30-5-143