The Amazon basin is the largest drainage basin in the world, about 7,050,000 square kilometres (2,720,000 sq mi), and accounts for approximately one-fifth of the world's total river flow.he Amazon River and its tributaries are characterized by extensive forested areas that become flooded every rainy season. Every year, the river rises more than 9 metres (30 ft), flooding the surrounding forests, known as varzea (vÃ¡rzea flooded). forests