The Amazon River, recognized for its immense size and the vast rainforest it sustains, is a freshwater river. It carries an extraordinary volume of water, shaping landscapes and supporting a diverse array of life.
Understanding Freshwater
Freshwater is defined by its low concentration of dissolved salts. Specifically, it contains less than 0.05% salt by weight, or less than 0.5 grams of salt per liter (0.5 parts per thousand). This distinguishes it from brackish water, which has a salt content between 0.5 and 30 grams per liter, and saltwater, which exceeds 30 grams per liter, typically around 35 grams per liter for seawater.
The Amazon’s Freshwater Sources
The Amazon River maintains its freshwater status due to a continuous influx of water from two primary sources: abundant rainfall across its sprawling basin and snowmelt from the Andes Mountains. The Amazon Basin receives significant annual rainfall. This precipitation generates a large amount of runoff that feeds into the river system. The eastern slopes of the Andes Mountains block warm, moist air, leading to heavy, persistent rainfall that contributes substantially to the Amazon’s tributaries. While the Andes comprise less than 10% of the basin area, they play a considerable role in the water cycle. This immense volume of incoming freshwater from both rainfall and snowmelt constantly dilutes any potential salinity. The Amazon’s vast network of over 1,000 tributaries further contributes to its flow, ensuring a consistent supply of fresh water.
The River’s Oceanic Frontier
Despite its immense discharge into the Atlantic Ocean, the Amazon River largely retains its freshwater characteristics even at its mouth. The river’s overwhelming discharge, averaging around 209,000 cubic meters per second, creates a massive freshwater plume that extends far into the ocean. This plume, which accounts for approximately 20% of the global freshwater input to oceans, pushes back the denser saltwater. The freshwater plume can extend 170 to 400 km in width and desalinate the ocean to depths of 50 to 100 meters, reaching over 1,800 km from the mouth. While tidal influences are present, especially near the mouth, the sheer volume of river discharge prevents saltwater intrusion into the main channel. This dynamic interaction creates the phenomenon of the pororoca, a powerful tidal bore that can form waves up to 4 meters high and travel up to 800 km upstream during spring tides.
Ecological and Human Importance
The Amazon’s freshwater nature is fundamental to its role as a global biodiversity hotspot and supports millions of people. It supports a vast diversity of aquatic life, including over 3,000 species of freshwater fish, making the Amazon Basin home to the largest share of freshwater species globally. Unique aquatic mammals like the pink river dolphin and the Amazonian manatee, the only exclusively freshwater manatee species, thrive in its waters; manatees, for instance, contribute to seed dispersal. For human populations along its banks, the Amazon River is a primary source of sustenance and connectivity, providing drinking water, facilitating transportation, and offering abundant food resources, particularly fish. The river system also contributes to regional and global climate regulation by influencing rainfall patterns and the water cycle, as the vast rainforest, sustained by its freshwater, releases billions of liters of water vapor daily, affecting weather patterns across continents.