Colo. Rev. Stat. § 30-5-121
The county of Eagle is hereby created and established, with the legal capacities and functions of other counties of this state, and with boundaries as follows:
Commencing at a point on the northern boundary of Lake county where the divide between the Eagle river and Ten Mile branches from and leaves the National range; thence along the summit of the said divide and the dividing ridge between the Piney and the Blue rivers to the southern line of Grand county; thence due west to a point six miles west of the 107th degree of west longitude; thence due south to the northern boundary line of Pitkin county; thence east along said boundary line to the summit of the National range; thence in an easterly direction along said summit of the National range to the place of beginning.
C.R.S. § 30-5-121