Brazil - Anavilhanas Ecological Station

Area: 350.018 hectares - Perimeter: 380 Km Climate: hot and humid tropical, with one to two month dry season. temp: 27,1 ° year average pluv: 2.075 mm total /year Altitude: 5 - 600 m. Geomorpholoqy: The Ecological station is constituted by a 100.000 ha. archipelago, of hundreds of elongated islands and canals within the Rio Negro riverbed, covered by inundated forests. Included to these are 250.000 ha. of not-flooded forests on the riversides. The river is 1 to 3 Km wide and 5 to 35 m deep. Sessonal variation of the water is +/- 10 m. Water pH varies between 3,7 and 5,5. The archipelago was created by accumulation of sediments resulting from the erosion of the Guyana Hills (Rio Branco) and intense flocculation with organic matter from the black acidic waters of Rio Negro. This process explains the islands peculiar composition (stilt and caolinite), form and disposition. Flocculation started in two parallel banks, meeting downstream and trapping sediment to form a delta-like area. Large basins and lakes between the islands, up to 20 m deep, are temporarily isolated during the dry season. Some channels are permanently open and have faster stream. Islands and channels location is changing overtime, due to the seasonal water cycles, periodic floods, acidity and debit fluctuations. Not-flooded area's of the margin separate minor affluent of the Rio Negro. They are tabular formations of yellow laterite soils and smooth altitude variation.