The Amazon River is the longest river in South America and the second-longest in the world (after the NILE RIVER), extending some 4,000 miles across the continent. The Amazon drains 2.7 million square miles, from near the Pacific Ocean on the inter-Andean plateau to the Atlantic Ocean. It flows through Peru, eastward across Brazil, and enters the Atlantic at the EQUATOR, where it discharges roughly a fifth of all the freshwater that drains into the oceans. The river has hundreds of tributaries; of these, 17 are more than 1,000 miles long. The main headwaters of the Amazon, the Ucayali and Marañón Rivers, have their source in the snows of the high ANDES MOUNTAINS of Peru. During the wet season, the Amazon varies in width from five to 40 miles. Near the Atlantic, it opens into an estuary about 150 miles wide, scattered into many branches by islands formed from silt deposits. Often called the “Ocean River” because of its vastness, the Amazon is highly navigable, and large ships can reach Iquitos in northeastern Peru, 2,300 miles from the sea. Before the European conquest of South America, the Amazon had no single name; its different sections were known to native peoples by specific names. The name of the river is probably derived from the native word amassona for “boat destroyer,” although others claim it was named by Europeans after the Amazons of Greek mythology.