The City and County of Denver is the capital and the most populous city of the U.S. state of Colorado. As Denver County, it is the second most populous county in Colorado right after El Paso County, which surpassed Denver County as of the 2010 Census. Denver is a consolidated city-county, located in the South Platte River Valley on the western edge of 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 12 miles (19 km) east of the foothills of the Rocky Mountains. Denver is nicknamed the Mile-High City because its official elevation is exactly one mile (1.6 km) or 5,280 feet (1,609.344 m) above sea level. The 105th meridian west of Greenwich passes through Union Station and is the temporal reference for the Mountain Time Zone.