The Klamath River is an American river that flows 263 miles (423 km) southwest through Oregon and northern California, cutting through the Cascade Range to empty into the Pacific Ocean. By average discharge, the Klamath is the second largest river in California after the Sacramento River. It drains an extensive watershed of almost 16,000 square miles (41,000 km2) that stretches from the high desert country of the Great Basin to the temperate rainforest of the Pacific coast. The upper basin once contained vast freshwater marshes that provided habitat for abundant wildlife, including millions of migratory birds; now it is largely agricultural, while the mountainous lower basin remains wild. The watershed is known for this peculiar geography, and the Klamath has been called "a river upside down" by the National Geographic Society.