The Amazon River culminates its journey by merging with the Atlantic Ocean. The world’s largest river by discharge volume, it releases an average of 209,000 to 230,000 cubic meters of water into the ocean per second. This immense outflow accounts for 15% to 20% of all freshwater discharged into the world’s oceans. This confluence represents a dynamic interaction between terrestrial and marine environments, shaping both the ocean and coastline. The sheer volume of water and sediment creates an expansive interface with the Atlantic.
The Expansive Delta and Estuary
The Amazon River does not have a single mouth, but forms a complex deltaic system before merging with the Atlantic. This estuarine zone features a network of channels, islands, and tidal flats. Marajó Island, the world’s largest fluvial island, is a prominent feature, comparable in size to Switzerland (40,100 to 42,000 square kilometers).
The main flow passes north of Marajó Island, while “furos” (natural channels) connect to the Pará River estuary south. This multi-channelled interface spans approximately 330 kilometers (205 miles) to the ocean, encompassing Marajó Island and the Pará River outlet. Despite its sediment load, the Amazon does not form a classic delta. Instead, ocean currents and a strong tidal bore disperse sediment offshore, forming a subaqueous delta rather than a protruding landform. The volume of freshwater ensures the estuary remains largely freshwater, even near the ocean.
The Dynamic Freshwater Plume
The meeting of the Amazon’s freshwater discharge with the Atlantic Ocean results in a dynamic freshwater plume, a distinct mass of river water that extends far beyond the coastline. This plume can stretch for hundreds to thousands of kilometers into the Atlantic, influencing the ocean’s properties over a vast area.
Less dense than saltwater, the freshwater forms a stratified layer, typically 3 to 15 meters thick near the continental shelf, which can deepen to 50 meters or more in the mid-Atlantic. This stratification creates a barrier layer that inhibits vertical mixing in the upper ocean, impacting heat exchange and nutrient distribution.
The plume’s movement is largely governed by ocean currents. The North Brazil Current initially transports the river water northwestward along the South American coast. Further offshore, the North Equatorial Countercurrent then carries portions of the plume eastward, sometimes reaching as far as the coast of Africa. This freshwater influx reduces surface salinity across wide oceanic regions, with drops of 3 to 8 practical salinity units (PSU) observed. It also influences sea surface temperature, leading to warmer surface layers due to reduced mixing.
A unique phenomenon marking this confluence is the “pororoca,” a powerful tidal bore. This wave, which can reach heights of up to 4 meters (13 feet), travels upstream into the river’s channels, sometimes for as much as 800 kilometers (500 miles). The pororoca occurs during the new and full moons, when the gravitational pull of the sun and moon combine to create exceptionally high tides, causing the incoming ocean water to forcefully reverse the river’s flow. Its approach is often heralded by a distinct roaring sound, giving it its indigenous name.
Unique Coastal Ecosystems
The interaction between the Amazon’s freshwater and the Atlantic’s marine environment fosters distinctive coastal ecosystems. Freshwater outflow and sediment create specialized habitats, particularly in the delta’s extensive mangrove forests. These mangroves thrive in very low salinity, unlike typical saltwater mangroves, and intermingle with freshwater floodplain species, forming a mixed vegetation zone. These forests are important for carbon sequestration and serve as habitats for terrestrial and aquatic species.
The Amazon basin has vast freshwater biodiversity, with over 3,000 fish species. Notable inhabitants include the Amazon river dolphin (boto or pink river dolphin), a freshwater species adapted to the delta’s murky, flooded forests. Freshwater stingrays, evolved from marine ancestors, also populate these waters.
The freshwater plume influences offshore marine life. It carries nutrients and organic matter from land, stimulating microbial activity and supporting phytoplankton blooms. These organisms form the foundation of the marine food web, affecting productivity across the western tropical Atlantic. A coral and sponge reef system, the Amazon Reef, was discovered beneath the plume, spanning over 1,000 kilometers. Its presence challenges assumptions about reef formation, thriving in turbid, low-salinity conditions near the river’s mouth.