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.