By middle school, most students have learned that the Amazon River is the mightiest of Earth’s rivers. Its mean discharge to the ocean of about 209,000 cubic meters per second is more than the combined discharges of the next six largest rivers in the world. Its farthest tributaries rise in the Andes a few hundred kilometers from the Pacific Ocean, and it drains an area of about 7 million square kilometers as it crosses South America to the Atlantic. The Amazon dwarfs North America’s largest river system, the Mississippi, which drains less than half the area of the Amazon and discharges about one-tenth as much water.
