Denver is the largest city and capital of the U.S. state of Colorado. Denver is a consolidated city and 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 or 5,280 feet (1,609.3 m) above sea level, making it one of the highest major cities in the United States. The 105th meridian west of Greenwich passes through Union Station and is the temporal reference for the Mountain Time Zone. With a 2012 estimated population of 634,265, Denver ranks as the 23rd most populous U.S. city.