Am'azon, a river of South America, the largest in the world, formed by the confluence of a great number of streams which rise in the Andes in Peru. It is formed by the union of two main branches, the Maranon or Tunguragua, and the Ucayali, or Apurimac The united Amazon, from the mouth of the Rio Negro to Tabatinga, is known as the Solimoens. It enters the Atlantic by a mouth 150 miles wide. From its junction with the Napo in Ecuador the Amazon flows due east; it is therefore almost wholly in the same latitude, which is not the case with any other large river. The total length of the river is between 3300 and 4000 miles. In its upper course navigation is interrupted by rapids, but from its mouth upward for a distance of about 3000 miles, mostly in Brazil, there is no obstruction. It receives the waters of about 200 tributaries, 100 of which are navigable, and seventeen of these are from 1000 to 2300 miles in length, the largest being the Madeira. The Amazonian water system affords some 15,000 miles of river suitable for navigation. The rapidity of the river is considerable, especially during the rainy season, from January to June, when it is subject to floods; but there is no great fall in its course. The tides reach up as far as 400 miles from its mouth. About the time of full moon a great tidal wave rushes into the mouth of the river with such force that it raises the water almost thirty feet. This singular phenomenon is called the bore, or the pororoca, by the natives. The river swarms with alligators, turtles and a great variety of fish. Steamers and other craft ply on the river, the chief center of trade being Para, at its mouth. The Amazon was discovered by Yanez Pincon in 1500, but the stream was not navigated by any European till 1540, when Francis Orellana descended it.