The City and County of Denver is the capital and the most populous city of the state of Colorado, in the United States. Denver is a consolidated city-county located in the South Platte River Valley on the High Plains just east of the Front Range of the Rocky Mountains. The Denver downtown district is located immediately east of the confluence of Cherry Creek with the South Platte River, approximately 15 miles (24 km) east of the foothills of the Rocky Mountains. The 105th meridian west of Greenwich passes through Union Station, making it the reference point for the Mountain Time Zone. The United States Census Bureau estimated that the population of Denver was 598,707 in 2008, making it the 24th most populous U.S. city. It is also the second-largest city in the Mountain West after Phoenix. Denver is the largest city in the Front Range Urban Corridor, a fast growing Combined statistical area stretching across eighteen counties in two states. The population of the Front Range Urban Corridor CSA is estimated at 4,251,663. The city has the tenth-largest central business district in the United States.