Anavilhanas Archipelago is the biggest freshwater archipelago in the world. It starts roughly 80 kilometers (50 miles) upstream from Manaus on the Rio Negro, smack in the middle of the Brazilian Amazon, and is made up of some 400 islands. At its widest, the river is measured to 27 kilometers (16.7 miles) wide. The water level can shift up to 15 meters (50 feet), thus creating a network of canals, dikes and sand dunes that only the most experienced fishermen and guides can safely cross.
The islands can vary from huge and permanent isles on which houses and hotels can be built, to only small banks of sand which disappear completely once the water level starts to rise. These island took on their current shape during the last Ice Age when water levels dropped in the Amazon basin and some of Rio Negro’s tributaries have brought and deposited sediments here from the Guyana Hills. Some huge ancient stones have remained standing and because the river’s waters were so shallow and slow, they weren’t able to remove the sediments that were building up around them.
These islands became so big that by the end of the Pleistocene, when the water levels began to rise again, the already existing islands have slowed the waters even further which in turn helped cement the sediments. This has helped maintain the island to this day despite the constant erosion done by the flowing water. New sediment is constantly brought in the by the Negro and Branco Rivers which keeps the Anavilhanas Archipelago in a constant transformation.
This entire area of islands is an important ecological system for a large number of fresh water turtles who use them as their nesting grounds during the dry season, from July to November, when the water levels are low. A large community of birds also call Anavilhanas Archipelago their home, living among the flooded forests between the islands. Manatees, giant beavers, pink Amazonian fresh water dolphins and the Arapaima (the largest fresh water fish in the world), all live here.