The Amazon River, the largest river system in the world by discharge volume, is definitively a freshwater river. Its continuous flow of water originates from rainfall and snowmelt across a vast basin, ensuring its chemical composition is characterized by a very low concentration of dissolved salts. This waterway carries approximately 20% of the total global freshwater transport that flows into the ocean. The scale of this system allows it to maintain its fundamental nature even as it interacts with the Atlantic Ocean.
Defining Freshwater and Salinity Levels
Water is categorized by its salinity, which is a measure of the concentration of dissolved salts, commonly expressed in parts per thousand (ppt). Freshwater is defined as having a salinity of less than 0.5 ppt. This low threshold is the standard used to classify the water found in most rivers, lakes, and streams.
In contrast, typical open ocean seawater has a much higher salinity, generally ranging between 33 and 38 ppt, with an average of about 35 ppt. The intermediate range, known as brackish water, occupies the zone between 0.5 ppt and approximately 30 ppt. This environment is often found in estuaries, where the mixing of river and ocean waters creates a fluctuating zone.
The Amazon’s Freshwater Plume and Ocean Interaction
The Amazon River’s discharge volume is so large that it effectively resists the formation of a typical, large brackish estuary. Instead of a gradual mixing zone, the river’s output acts like a continuous jet stream into the Atlantic Ocean. This constant flow, estimated at around 200,000 cubic meters of water per second, prevents significant saltwater intrusion far inland.
Upon meeting the ocean, the river water creates the freshwater plume, a massive lens of low-salinity water that fans out across the Atlantic surface. Because freshwater is less dense than saltwater, it floats on top of the heavier ocean water, creating a stratified layer. This distinct, low-salinity surface layer can be 3 to 10 meters thick and 80 to over 400 kilometers wide.
The influence of this plume extends hundreds of miles from the coast, sometimes reaching over 500 kilometers out to sea. Carried by ocean currents, the low-salinity water is often deflected northwestward, with effects traced into the distant Caribbean Sea. This river output significantly impacts the surrounding marine environment, influencing regional weather patterns, ocean currents, and nutrient distribution for marine life.
The plume’s surface salinity remains remarkably low, with measurements far out in the Atlantic sometimes showing salinity values around 25 practical salinity units. This hydrological dominance allows the coastal region to remain predominantly freshwater despite its direct connection to the sea. The volume and velocity of the river water overwhelm the ocean’s ability to push salt water upriver.
The Phenomenon of the Tidal Bore
The river’s interaction with the ocean includes a temporary effect known as the tidal bore, or “Pororoca,” meaning “great roar” in the local Tupi language. This event occurs when the incoming ocean tide, particularly during strong spring tides, encounters the shallow, outgoing flow of the river. The collision generates a breaking wave that surges upstream.
The Pororoca can reach heights of up to 4 meters (13 feet) and travel inland at speeds between 16 to 24 kilometers per hour (10 to 15 mph). This wave has been documented to travel as far as 800 kilometers (500 miles) up the river and its tributaries. The force of the bore causes significant erosion along the riverbanks, uprooting trees and reshaping channels.
The tidal bore is a hydrodynamic event, meaning it is a wave of water, not a permanent change in the river’s composition. While the ocean tide forces water upstream and momentarily reverses the surface current, the intrusion is transient. It does not fundamentally alter the underlying freshwater status of the Amazon or redefine it as a saltwater environment.