The Amazon River and Flooded Forests ecoregion contains an extremely high number of freshwater animal species, including two species of freshwater dolphins. The ecoregion is characterized by extensive forested areas that become flooded during each rainy season. These "flooded forests" in the Amazon region are the most extensive of this habitat type in the entire world. The cycle of flooding, and the annual regeneration that it produces, represent a globally outstanding ecological phenomenon. The entire Amazon basin contains the largest number of freshwater fish species in the world, estimated at more than 3,000 species. Every year the Amazon River rises more than 30 feet (9 m) and floods surrounding forests. These rising waters replenish nutrients in floodplain areas and regenerate floodplain lakes, floating meadows, and other seasonal habitats. Freshwater fish move into these flooded forests and often feed on the fruits that drop from the trees. The Amazon is the second longest river in the world (after the Nile), stretching 4,000 miles (6,400 kilometers). But it's the largest when it comes to volume. The Amazon contains 20 percent of the Earth's fresh water, which it carries from high up in the Andes Mountains to the Atlantic Ocean. Along the way, it picks up rich soil that flows into dozens of streams and small rivers. The Amazon Basin has two freshwater systems, the igapó (blackwater) and the varzea (whitewater). The varzea contains a large amount of minerals that fertilize land and help trees and other vegetation grow.