09-4-4533. The Anavilhanas, the name given to around 350 forested islands in Brazilís Rio Negro, form the worldís largest inland archipelago. Covering 1,000 square kilometers of Amazonia, they start 80 kilometers north-west of Manaus and stretch some 400 kilometers up the Rio Negro as far as Barcelos. Their formation dates back to the last Ice Age when changes in the flows of rivers entering the Rio Negro produced accumulations of sediment which, over time, resulted in sandbars and islands. Since water levels change with the seasons by as much as 20 meters, the Anavilhanas are themselves ever-changing, with channels, sandbars and lagoons appearing during the dry season and some small islands vanishing when waters rise. Many of the larger islands, though, are self-contained parcels of rain forest. Brazil. May 2009.