II.

Course and Physical Environment

The major headstreams of the Amazon are the Ucayali and Marañón rivers. They both rise in the permanent snows and glaciers of the high Andes, and rush through waterfalls and gorges, following parallel courses north before joining near Nauta, Peru. From this confluence the main trunk of the Amazon flows in a generally eastern direction to the Atlantic Ocean. The Amazon enters the Atlantic through a broad estuary, roughly estimated at 240 km (about 150 mi) in width. Here deposits of sediment have formed a maze of islands that separate the river into branches. The mouth of the main stream is 80 km (50 mi) wide. This branch, known as the Pará, is separated from a smaller branch by Marajó Island, which has an area (including salt marshes) of more than 36,000 sq km (14,000 sq mi). During new and full moon a tidal bore, or wave front from the ocean, sweeps some 650 km (more than 400 mi) upstream at speeds in excess of 65 km/h (40 mph). This phenomenon often causes waves up to 5 m (16 ft) in height.

The Amazon watershed includes the largest and wettest tropical plain in the world, and the largest rainforest. Heavy rains drench much of the lowland region throughout the year, but especially between January and June. Seasonal variation in rainfall is reflected by the width, rate of flow, and discharge volume of the river, and the annual average in the region ranges from 2,000 mm (79 in) to 3,000 mm (118 in). During the months of maximum precipitation, broad areas traversed by the Amazon are subject to severe floods. In Brazil the width of the river ranges between 1.6 and 10 km (1 and 6 mi) at low stage but expands to 48 km (30 mi) or more during the annual floods; the rate of flow ranges between 2.4 and 8 km/hr (1.5 and 5 mph); and the crest of the water at flood time often rises 15 m (50 ft) above the norm. To drain the vast mass of water, the Amazon has carved a deep bed in the plain through which it flows. In one sector near Óbidos, Brazil, the bed is more than 91 m (300 ft) below the average surface level of its water.

Because of its vastness, annual floods, and navigability, the Amazon is often called the Ocean River. The total number of its tributaries is as yet uncounted, but more than 200 are in Brazil alone. Seventeen of the largest known tributaries are more than 1,600 km (1,000 mi) in length. The Amazon proper is navigable to ocean liners of virtually any tonnage for two thirds of its course. Transatlantic ships call regularly at Manaus, nearly 1,600 km (1,000 mi) upstream; and ships of 3,000 tons can reach Iquitos, Peru, 3,700 km (2,300 mi) from the river's mouth, the farthest point from sea of any port serving ocean traffic. River steamers of more modest tonnage can navigate on more than 100 of the larger tributaries.