Kauai's Waimea River rises in a wet plateau of the island's central highlands, in the Alaka'i Swamp, the largest high-elevation swamp in the world, and flows south, passing through the spectacular 3,000-foot-deep (910 m) Waimea Canyon, known as the "Grand Canyon of the Pacific."

The highest peak on this mountainous island is Kawaikini at 5,243 feet (1,598 m) above sea level. A rain gauge placed nearby marks one of the wettest spots on earth, with an annual average rainfall of 460 inches (11.7 m). This high rainfall makes reaching the summit difficult on most days.

Not only does Kauai hold records for average yearly precipitation, it also holds a record in hourly precipitation. During a storm on January 24-25, 1956, a rain gauge at Kauai's former Kilauea Sugar Plantation recorded twelve inches (305 mm) of precipitation in just 60 minutes--but the actual measurement would have been higher because the rain gauge overflowed. A taller gauge might have allowed Kauai to exceed Holt, Missouri's world-record rainfall of 12 inches in 42 minutes on June 22, 1947.

In 1835, the first sugarcane plantation was founded on Kauai, and for the next century the industry would dominate the local economy. Most of that land is now used for ranching.

With an area of 562.3 square miles (1,456.4 km2), it is the fourth-largest Hawaiian island and the 21st largest island in the United States.