A Riverine Labyrinth
The Anavilhanas Archipelago is a product of fluvial dynamics on a massive scale. This group of 400 islands exists because of a geological meeting of the waters. The archipelago's islands are not formed by the Rio Negro itself, which is a blackwater river with very little suspended sediment. Instead, the sediment comes from the Rio Branco, a tributary that carries eroded material from the Guyana Highlands and deposits it into the Rio Negro just upstream. This sediment load, combined with the slow-moving, lake-like widening of the Rio Negro, forms these countless elongated islands.
The entire system is in a state of flux, governed by the annual flood pulse. Between the wet and dry seasons, the water level of the Rio Negro can vary by 8 to 12 meters. During the wet season, from roughly March to August, the islands are submerged, creating a vast flooded forest known as igapó. As the waters recede from September to February, expansive white-sand beaches emerge along the islands' edges. This constant cycle of erosion and deposition means the islands slowly change shape, size, and position over time.
An Acidic Aquatic World
The unique chemistry of the Rio Negro defines the archipelago's biology. The river’s water is acidic, with a pH that can range from 3.8 to 4.9, and is stained dark by tannins and humic acids from decaying plant matter. This chemical composition creates an environment that supports a specialized collection of life while inhibiting others—for instance, the acidic water significantly reduces the population of mosquitoes.
This blackwater environment hosts the largest array of electric fish in the world. The park protects threatened species such as the Amazonian manatee, the black caiman, and two species of river dolphin, including the pink river dolphin, or boto. The forests of the islands and surrounding mainland shelter jaguars, giant otters, and tapirs. The entire Anavilhanas National Park covers about 350,000 hectares and is part of the Central Amazon Conservation Complex, a UNESCO World Heritage Site that forms the largest protected area in the Amazon Basin.