Rainforests are forests characterized by high rainfall, with definitions based on a minimum normal annual rainfall of 1750-2000 mm (68-78 inches). The monsoon trough, alternatively known as the inter tropical convergence zone, plays a significant role in creating the climatic conditions necessary for the Earth's tropical rainforests.

Around 40% to 75% of all biotic species are indigenous to the rainforests. It has been estimated that there may be many millions of species of plants, insects and microorganisms still undiscovered in tropical rainforests. Tropical rainforests have been called the "jewels of the Earth" and the "world's largest pharmacy", because over one quarter of natural medicines have been discovered there. Rainforests are also responsible for 28% of the world's oxygen turnover, sometimes misnamed oxygen production, processing it through photosynthesis from carbon dioxide and consuming it through respiration.

The undergrowth in a rainforest is restricted in many areas by the poor penetration of sunlight to ground level. This makes it easy to walk through undisturbed, mature rainforest. If the leaf canopy is destroyed or thinned, the ground beneath is soon colonized by a dense, tangled growth of vines, shrubs and small trees, called a jungle. There are two types of rainforest, tropical rainforest and temperate rainforest. read more

The Amazon River in South America is the second longest river in the world and by far the largest by water flow with an average discharge greater than the next seven largest rivers combined (not including Madeira and Rio Negro, which are tributaries of the Amazon). The Amazon, which has the largest drainage basin in the world, about 7,050,000 square kilometers (2,720,000 sq mi), accounts for approximately one-fifth of the world's total river flow.

In its upper stretches, above the confluence of the Negro River, the Amazon is called Solimões in Brazil; however, in Peru, Colombia and Ecuador, as well as the rest of the Spanish-speaking world, the river is generally called the Amazon downstream from the confluence of the Marañón and Ucayali rivers in Peru. The Ucayali-Apurímac river system is considered the main source of the Amazon.The width of the Amazon varies between 1.6 and 10 kilometers (1.0 and 6.2 mi) at low stage, but expands during the wet season to 48 kilometres (30 mi) or more. The river enters the Atlantic Ocean in a broad estuary about 240 kilometres (150 mi) wide.

The mouth of the main stem is 80 kilometres (50 mi).Because of its vast dimensions, it is sometimes called The River Sea. At no point is the Amazon crossed by a bridge. This is not because of its huge dimensions; in fact, for most of its length (which is 4,080 miles), the Amazon's width is well within the capability of modern engineers to bridge. However, the bulk of the river flows through tropical rainforest, where there are few roads or cities, limiting the need for crossings.

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