Colo. Rev. Stat. § 30-5-104
All that part of Arapahoe county as is included within the following described boundaries shall be set apart and is hereby established as a county, to be called the county of Arapahoe, the boundaries are as follows, to wit:
Beginning at the southwest corner of Arapahoe county; thence east to the intersection of the east boundary line of range fifty-seven west, with the south boundary line of Arapahoe county; thence north to the intersection of the east boundary line of range fifty-seven and south boundary line of township three in said range; thence west to the point of intersection of south boundary line of township three in range sixty-seven west and the east boundary line of the city of Denver as now constituted; thence following the eastern, southern and western boundary lines of the city of Denver as the same are constituted at the time this section goes into effect to a point where said boundary line of the city of Denver intersects the west boundary line of the county of Arapahoe; thence south to the southwest corner of Arapahoe, or place of beginning.
C.R.S. § 30-5-104