Austin is the capital of the U.S. state of Texas and the seat of Travis County, although portions extend into Hays and Williamson counties. It is the 11th-most populous city in the U.S. and the 4th-most populous in Texas. It is the fastest growing large city in the United States and the second most populous capital city after Phoenix, Arizona. As of the U.S. Census Bureau's July 1, 2015 estimate, Austin has a population of 931,830. Located in Central Texas in the foothills of Texas Hill Country, the city is home to numerous lakes, rivers, and waterways including Lady Bird Lake, Barton Springs, McKinney Falls, the Colorado River, Lake Travis, and Lake Walter E. Long. It is the cultural and economic center of the Austin–Round Rock metropolitan area, which had an estimated population of 2,056,405 as of July 1, 2016.