Beautiful Images Of Boulder County, Colorado. From 4,880 feet to 14,255 feet in Elevation. Boulder County was one of the original 17 counties created by the Territory of Colorado on 1861-11-01. The county was named for Boulder City and Boulder Creek, so named because of the abundance of boulders in the area. Boulder County retains essentially the same borders as in 1861, although a 27.5 square miles (71.2 km²)of its southeastern corner and its approximate population of 40,000 became part of the City and County of Broomfield in 2001.