Colo. Rev. Stat. § 30-5-158
So much of the county of Garfield as is included within the following described boundaries shall be set apart and is hereby established as a county, with the legal capacities and functions of other counties of this state, to be called the county of Rio Blanco:
Commencing at a point where the line between townships four and five south intersects the line between the state of Colorado and the territory of Utah; thence east on said township line to the southeast corner of township four south, range one hundred west; thence north three miles to the southeast corner of section thirteen, township four south, range one hundred west; thence east to the corner of sections fifteen, sixteen, twenty-one and twenty-two in township four south, range ninety-four west; thence north nine miles to the northeast corner of section four, township three south, range ninety-four west; thence east to the northeast corner of township three south, range ninety west; thence north six miles to the northeast corner of township two south, range ninety west; thence east to the eleventh guide meridian west; thence north on said eleventh guide meridian line to the base line; thence east on said base line to the southeast corner of township one north, range eighty-nine west; thence north six miles to the northeast corner of said township; thence east to the east boundary line of Garfield county; thence north to the south boundary line of Routt county; thence west on the south boundary line of Routt county to the state line; thence south on said state line to place of beginning.
C.R.S. § 30-5-158