The Americas are lands in the Western Hemisphere that comprise the continents of North and South America, along with their associated islands. The topography is dominated by the American Cordillera, a chain of mountains that run the length of the west coast. The flatter eastern side of the Americas is dominated by river basins, such as the Amazon, Mississippi, and La Plata. Extending 14,000 km (8,699 miles) in a north-south orientation, the climate and ecology varies strongly across the Americas, from arctic tundra of Northern Canada, Greenland, and Alaska, to the tropical rain forests in Central America and South America. When the continents joined three million years ago, the Great American Interchange resulted in many species being spread across the Americas, such as the cougar, porcupine, and hummingbirds.