Kauaʻi or Kauai is geologically the oldest of the main Hawaiian Islands. With an area of 562.3 square miles (1,456.4 sq km), 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 (169 km) 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 the island was 67,091, with the largest town by population being Kapaʻa.
Kauaʻi's origins are volcanic, the island having been formed by the passage of the Pacific plate over the Hawaii hotspot. At approximately six million years old, it is the oldest of the main islands. The highest peak on this mountainous island is Kawaikini at 5,243 feet (1,598 m).
The second highest peak is Mount Waiʻaleʻale near the center of the island, 5,148 feet (1,569 m) above sea level. One of the wettest spots on earth, with an annual average rainfall of 460 inches or 1170 cm, is located on the east side of Mount Waiʻaleʻale. The high annual rainfall has eroded deep valleys in the central mountains, carving out canyons with many scenic waterfalls. On the west side of the island, Waimea town is located at the mouth of the Waimea River, whose flow formed Waimea Canyon, one of the World's most scenic canyons, and which is part of Waimea Canyon State Park. At 3,000 feet (914 m) deep, Waimea Canyon is often referred to as "The Grand Canyon of the Pacific". Kokeo Point lies on the south side of the island. The Na Pali Coast is a center for recreation in a wild setting, including kayaking past the beaches, or hiking on the trail along the coastal cliffs. The headland, Kuahonu Point, is on the south-east of the island.