The Anavilhanas and Mariuá Archipelagos: Fluvial Wonders from the Negro River, Amazon Basin

Abstract

With a mean annual discharge of *29,000 m 3 s −1 , the Negro River ranks as the sixth in the world in terms of water discharge and is the second largest tributary of the Amazon. The Mariua and Anavilhanas are two huge, fascinating archipelagoes of the Negro River that sustain the largest flooded "igapó" forest systems in the world and rich fish diversity. This chapter presents how hydro-geomorphology and changes in environmental conditions controlled the formation and functioning of these two anabranching reaches in the Negro River. The present hydrosedimentary dynamics is not compatible with the existing morphology, which is product of a Middle-Late Holocene history. The very low amount of suspended sediment transported by the Negro is not sufficient to construct the intricate islands and floodplain. This condition produces a permanent non-equilibrium stage that controls not only river geomorphology but also vegetation distribution. As a product of long history, the Negro River basin with its wonderful pristine fluvial archipelagos and the "igapó" forests faces their most dangerous enemy: The irrecoverable destruction of the Amazon Rivers by the environmentally irresponsible dam construction planned by the Brazilian government.

Analysis of Suspended Sediment in the Anavilhanas Archipelago, Rio Negro, Amazon Basin

“…These results reinforce the idea that the Anavilhanas, which have little erosion process, still has areas with active deposition and an environment that continues to slowly form. However, these dynamics are different from the idea of geomorphological heritage proposed by Latrubesse and Stevaux [9].…”

“…As described by Latrubesse and Stevaux [9], the debate about the origin of the Anavilhanas islands started in the 1970s with Tricart [59], who modeled the formation of the Archipelago from a delta model in the Negro River, as a result of the ascent sea level in the middle Holocene, after the Last Glacial Maximum (LGM). Subsequently, Leenher and Santos [60] indicated the occurrence of flocculation of suspended sediments in the Branco River by the waters of the Negro River, and that this process created deposits on the islands of the Archipelago.…”

“…In a complementary way, Sioli [61] pointed out that the islands were formed as a consequence of the sediments transported by the Branco River. However, some of these ideas are not confirmed, especially the one about the Branco River contributions, since the Mariuá Archipelago, located above the mouth of the Branco River, has features and geomorphological forms similar to the Anavilhanas, but does not have a current source of sediments, such as the Branco River [9,11].…”

Phylogeography and population dynamics of Antbirds (Thamnophilidae) from Amazonian fluvial islands

“…Stronger indication of population structure among archipelagos in the mtDNA than in the nuDNA may reflect recent secondary contact of previously isolated lineages. Historical isolation of populations may have originated the distinct mtDNA lineages, while the recent re‐establishment of gene flow due to the formation and expansion of island environments in the lower Negro river during the Holocene may have resulted in the lack of structure in the microsatellite data (Latrubesse & Franzinelli, ; Latrubesse & Stevaux, ). Although this explanation agrees with the available geological information on island evolution, the alternative explanation of recent isolation with incomplete lineage sorting being stronger in the nuclear data due to larger population size cannot be ruled out with the current dataset.…”

Downstream grain‐size changes associated with a transition from single channel to anabranching

“…This question is particularly relevant for rivers where geologically controlled narrow reaches with single channels are interspersed with reaches characterized by wide floodplains and many anabranches. This transition has been documented in different climatic and tectonic settings, for instance in Australia (Knighton & Nanson, , ; Gibling et al ., ; Tooth & Nanson, ), Africa (Latrubesse, ; Runge, ) and South America (Latrubesse and Franzinelli, ; Latrubesse & Stevaux, ). This paper investigates how grain‐size changes between adjacent reaches with strongly contrasting river pattern, where the river planform is controlled by the geological setting; a topic that is poorly understood and remains understudied.…”