Anavilhanas is the world's largest inland archipelago. Beginning some 50 miles (80 kilometers) west (upstream) of Manaus in the Rio Negro in the center of the Brazilian Amazon, it forms a complex of 350 islands. Water levels vary by as much as 50 feet (15 meters), making for an ever-changing matrix of channels, levees, and sandbars that only the most experienced guides and fishermen can negotiate with certainty. The islands range from substantial, permanent, forest-topped entities, which are large and stable enough to support luxury hotels, to remote sandbars that completely disappear at high water.
The islands achieved their current form during the last ice age when water levels fell throughout the Amazon basin and changes in hydrology caused several of the Rio Negro's feeder rivers to deposit an excess of sediments. Several huge ancient rocks stood proud from the now shallow river and, in its reduced state, the Rio Negro was not able to flush away the deposits that built up around them, eventually forming islands.
These islands became so extensive that, when river levels rose again at the end of the Pleistocene, the newly extended island chain continued to slow down the river waters, causing suspended sediment to drop out. This maintained the islands and a new permanent habitat was formed, one that trapped almost all the sediment in the Rio Negro and so renewed itself against the erosion of the water. This process continues today, fed primarily by the sediment-rich Rio Branco and also by the Rio Negro. Though sediment concentration is low in the Rio Negro, it is such a large river that there is enough material to maintain the ever-changing complex of islands.
The majority of the archipelago lies within the Anavilhanas Archipelago Ecological Station. This is an area of major importance for several species of freshwater turtle that nest on sandy islets during the low water season (July to November), and for the various species of birds that are adapted to living in the archipelago's flooded forests. Anavilhanas is also a prime haunt of manatees, giant river otters, the elusive Amazon pink river dolphin, and the pirarucu, the world's largest freshwater fish, which can grow up to 10 feet (3 meters) in length