South America

The Amazon basin is the largest river basin found in the world, covering an area of about 2.73 million sq. mi (7 million sq. km). The second largest river basin, which is the basin of the River Zaire in the African Congo, is not even half as large. The water resources of the area are spectacular; the volume of water which flows from the basin into the sea is about 11% of all the water drained from the continents of the earth. The greatest flow occurs in July, and the least is in November. While there are many rivers flowing through the basin, the most important and well-known of these is the Amazon. The width of the Amazon ranges from about 1 mi (1.6 km) to as wide as 8-10 km (5-6 mi), and although it is usually only about 20-40 ft (6-12 m) deep, there are narrow channels where it can reach a depth of 300 ft (100 m).

The Amazon basin was once an enormous bay, before the Andes were pushed up along the coasts. As the mountain range grew, they held back the ocean and eventually the bay became an inland sea. This sea was finally filled by the erosion of the higher land surrounding it, and finally a huge plain, crisscrossed by countless waterways, was created. Most of this region is still at sea level, and is covered by lush jungle and extensive wetlands. This jungle region has the largest extent of any rain forest in the world, and is thought to have upwards of 100 different species per square kilometer. Despite the profusion of life that abounds here, the soil is not very rich; the fertile regions are those which receive a fresh layer of river silt when the Amazon floods, which occurs almost every year.