Kauaʻi or Kauai is geologically the oldest of the main Hawaiian Islands. With an area of 562.3 square miles, it is the fourth largest of the main islands in the Hawaiian archipelago, and the 21st largest island in the United States. Known also as the "Garden Isle", Kauaʻi lies 105 miles across the Kauaʻi Channel, northwest of Oʻahu. This island is the site of Waimea Canyon State Park. The United States Census Bureau defines Kauaʻi as Census Tracts 401 through 409 of Kauaʻi County, Hawaiʻi, which is all of the county except for the islands of Kaʻula, Lehua, and Niʻihau. The 2010 census population of Kauaʻi was 67,091, with the largest town by population being Kapaʻa.