Reading Section Part B

Look at a map of South America, and you will notice that the Amazon River generally follows a line just south of the equator. It flows across the northern part of South America, from the Andes in Peru, through Colombia and Brazil, to the Atlantic Ocean in the east. The region drained by this long river is vast, extending several hundred kilometers north and south of the Amazon River itself. This region, also known as the Amazon basin, is mainly in Brazil. The Amazon region of Brazil is covered by the world's largest tropical rainforest, containing beautifully colored parrots, many species of monkeys, as well as other animals and plants not found anywhere else. Most of the rainforest is impassable, except for along the rivers, streams, and few highways. As the Amazon River flows eastward, it is joined by many branches, and grows deep and wide.

What is the main topic of this passage?