County | FIPS code | County seat | Established | Formed from | Etymolgy | Population | Area | Map | |
Adams County | 001 | Brighton | 1902-11-15 | Split fromArapahoe County. | Named in honor ofAlva Adams, the 5th, 10th, and 14thGovernorof the State of Colorado. | 441,603 | 1,182.29sqmi (3,062km) | | |
Alamosa County | 003 | Alamosa | 1913-03-08 | Split fromCostilla CountyandConejos Countycounties. | Named for thecottonwood treeswhich grow along theRio Grandeand its tributaries.Alamosais a Spanish word for acottonwood grove. | 15,445 | 723.21sqmi (1,873km) | | |
Arapahoe County | 005 | Littleton | 1861-11-01 | Created as one of the 17 original counties of theTerritory of Colorado. Renamed South Arapahoe County for the five months from November 15, 1902 to April 11, 1903. | Named for predecessorArapahoe County, Kansas Territory, which in turn was named for theArapaho NationofNative Americans. | 572,003 | 804.41sqmi (2,083km) | | |
Archuleta County | 007 | Pagosa Springs | 1885-04-14 | Split fromConejos County. | Named in honor ofColorado State SenatorAntonio D. Archuleta. | 12,084 | 1,354.53sqmi (3,508km) | | |
Baca County | 009 | Springfield | 1889-04-16 | Split fromLas Animas County. | Named in honor of pioneer andColorado territorial legislatorFelipe Baca. | 3,788 | 2,558.48sqmi (6,626km) | | |