The Amazon flows into the Atlantic Ocean forming an estuary that is 240 kilometres wide. It discharges so much water into the Atlantic, that, more than 160 kilometres into the open sea, opposite the river mouth, you could still drink freshwater from the ocean.

The Amazon River’s freshwater discharge is so massive that its "plume" can be detected up to 400 kilometers (250 miles) out into the Atlantic Ocean. Because freshwater is less dense than saltwater, it floats on the surface, creating a "freshwater ring" that covers nearly 1.3 million square kilometers. This plume significantly alters the ocean's salinity and color; sailors have famously reported being able to drink fresh water from the ocean's surface while still far from the sight of land. In 2026, climate scientists monitor this plume closely, as it carries vast amounts of nutrients that fuel massive blooms of phytoplankton and "sargassum" seaweed, which can reach as far as the Caribbean. The sheer force of the Amazon—discharging roughly 200,000 cubic meters of water per second—essentially creates a "river within the sea" that influences the marine biology and chemistry of the entire North Atlantic region.