The Columbia River pours more water into the Pacific Ocean than any other river in North or South America. In its course to the Pacific Ocean, the Columbia flows through four mountain ranges—the Rockies, Selkirks, Cascades, and coastal mountains—and drains . The mainstem of the Columbia rises in Columbia Lake on the west slope of the Rocky Mountain Range in Canada. Its largest tributary, the Snake, travels from its source in Yellowstone National Park in Wyoming before joining the Columbia. When Lewis and Clark explored the region in the early 19th century, huge numbers of fish (salmon) returned to spawn every year. The multitudes of this fish are almost inconceivable, Clark wrote in the autumn of 1805. At that time, the Columbia and its tributaries provided of pristine river habitat.