The Amazon River: Origins, Course, and Characteristics

The Upper Amazon and Its Tributaries

The Upper Amazon, also known as the Solimões, is a region encompassing the river systems and floodplains within Brazil, Peru, Ecuador, Colombia, and Venezuela. The Amazon River itself, flowing mainly through Brazil and Peru, also delineates part of the Colombia-Peru border. It receives water from several significant tributaries, such as the Putumayo, Caquetá, Vaupés, and Guainía, which join either the Marañón and Ucayali or enter the Amazon directly. The river's path is characterized by anabranches and a complex network of natural canals, forming numerous islands within the flooded igapó forests.

The Lower Amazon and Its Estuary

The Lower Amazon is marked by the meeting of the Rio Negro's dark waters with the sandy-hued Rio Solimões, where they flow alongside each other for over six kilometers without mixing. The lower reaches of the river are believed to have been an ancient gulf of the Atlantic Ocean, as suggested by the presence of cliffs near Óbidos. The Amazon's estuary is expansive, with the Pará River and the oceanic front of Marajó island contributing to its breadth, which spans up to 325 kilometers when these features are included. The estuary's influence extends into the tropical north Atlantic, affecting the region's salinity, pH, light penetration, and sedimentation patterns.

The Amazon River's Bridgeless Span and Contested Length

The Amazon River, despite its enormity, has no bridges crossing its full width. This absence is not due to technical challenges, as engineering solutions exist, but rather because the river traverses the sparsely populated Amazon Rainforest, where infrastructure such as roads and cities is minimal. The length of the Amazon has been a topic of debate among geographers, with some considering it the world's second-longest river after the Nile. The Amazon's length has been reported to range from 6,275 to 6,992 kilometers, while the Nile's length varies between 5,499 and 7,088 kilometers. These discrepancies arise from different methodologies for identifying the source and mouth, the scale of measurement, and the techniques used to measure length. In 2008, the Brazilian Institute for Space Research (INPE) posited that the Amazon might be 140 kilometers longer than the Nile. However, this claim, among others, remains contested, and the precise length of the Amazon continues to be a subject of ongoing scientific exploration and debate.