Colo. Rev. Stat. § 24-32-109
No annexation, consolidation, merger, detachment of any area, new incorporation, or dissolution of an existing municipality shall be effective until notice of the completion of such action with a legal description accompanied by a map of the area concerned is filed in duplicate by the municipality with the county clerk and recorder of the county in which the annexation, consolidation, merger, detachment, incorporation, or dissolution takes place. In case such action effects a change in county boundaries, the same shall be filed with the county clerk and recorder of each county affected. A certified duplicate copy of any annexation, consolidation, merger, detachment, incorporation, or dissolution shall be filed with the division of local government by the county clerk and recorder of the county.
C.R.S. § 24-32-109
For effect of failure to file, see § 31-12-113(2).