A river basin, also known as a drainage basin or watershed, is the expanse of land where all surface water collects and drains into a single river, stream, lake, or other body of water. This area acts like a natural funnel, channeling precipitation and runoff from a defined region toward a common outlet. River basins are fundamental to life on Earth, forming the circulatory system for freshwater and supporting terrestrial and aquatic ecosystems. They are the source of water supply for human populations, agriculture, and industry.
The Amazon River Basin and Key Metrics
The largest river basin in the world is the Amazon River Basin, located in South America. Its immense size is confirmed by quantifiable metrics that establish its dominance over all other basins. The total surface area of the Amazon drainage basin spans approximately 7 million square kilometers (2.7 million square miles), covering roughly 40% of the South American continent.
The basin’s defining feature is the volume of water it moves, making the Amazon River the largest in the world by discharge. The river pours an average of 215,000 to 230,000 cubic meters of water into the Atlantic Ocean every second. This discharge is greater than the next seven largest independent rivers combined. This single river system is responsible for approximately 20% of all the freshwater discharged into the world’s oceans.
The main stem of the Amazon River is recognized as one of the two longest rivers globally, with a length of about 6,400 kilometers (4,000 miles). The basin’s scale is so vast that two of its tributaries, the Madeira and the Rio Negro, would rank among the top ten largest rivers in the world if measured independently.
Geography, Hydrology, and Global Significance
The Amazon River Basin is an international entity, spanning nine countries across South America. The majority of the area—roughly 60%—lies within Brazil. Other nations contributing to the basin include Peru, Colombia, Venezuela, Ecuador, Bolivia, Guyana, Suriname, and French Guiana. The basin is geographically defined by the Andes Mountains to the west, where the river’s most distant sources originate, and the Brazilian and Guiana Highlands to the south and north.
The influence of the Andes is fundamental to the basin’s hydrology, as the mountain range traps atmospheric moisture and provides the massive runoff that feeds the river system. This process results in an extensive network of over 1,100 tributaries, with 17 of them stretching more than 1,500 kilometers long. The basin is characterized by extreme seasonal flooding, where the river can rise over nine meters in the wet season. This expansion increases the water-covered area from an average of 110,000 square kilometers to as much as 350,000 square kilometers.
This immense water system supports the Amazon rainforest, which is the largest tropical rainforest in the world and covers the majority of the basin. The region holds a massive amount of the world’s biodiversity, containing approximately 10% of the globe’s known species and an estimated 25% of the world’s terrestrial species. The rainforest plays a significant role in global climate regulation by absorbing vast amounts of carbon dioxide and influencing atmospheric moisture.
Contextualizing the Size Against Other Major Basins
The size of the Amazon River Basin is best understood when compared to the world’s other major drainage systems. Ranking second is the Congo River Basin in Central Africa, which covers roughly 3.7 to 4 million square kilometers. Since the Amazon Basin is almost twice this size, the comparison highlights the magnitude of the South American system.
The third largest basin is the Nile River Basin in northeastern Africa, covering approximately 3.2 to 3.4 million square kilometers. The Mississippi River Basin in North America is comparable in area to the Nile, covering around 3.22 million square kilometers.
In terms of both land area and water discharge volume, the Amazon Basin remains the giant of the world’s hydrological features. Its extensive reach across the continent and the powerful flow of the river solidify its status as the largest and one of the most ecologically important river basins.