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 from the Andes in Peru to the Atlantic in the east, across the northern part of South America. The region drained by this long river is vast, extending several hundred miles north and south of the Amazon, and into Venezuela, Columbia, Ecuador, Peru, and Bolivia. However, the greater part of the Amazon region, or basin, is in Brazil. The Amazon region of Brazil is covered by the world's largest tropical rain forest. This jungle is almost completely impassable, except along the rivers and streams. In the dark forests there are beautifully colored parrots, noisily chattering monkeys, and many other strange animals and plants. The only highway across the northern part of Brazil is the Amazon River. As it flows eastward it is joined by many branches, and grows deep and wide.

What is the main topic of this passage?