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Fluvial dynamics in the Andean foreland; megafans and mechanisms for biodiversity in aquatic biota


, : Fluvial dynamics in the Andean foreland; megafans and mechanisms for biodiversity in aquatic biota. Abstracts with Programs - Geological Society of America 31(7): 189

River dynamics, and hydrologic interactions with tectonic elements (foreland basins, forebulges, etc. ), provide mesoscale biogeographic vicariance mechanisms, in contrast to macroscale controls such as mountain and isthmus building. Key features in the Andean foreland are nested megafans, hundreds of km in radius (10 (super 4) -10 (super 5) km (super 2) ), transverse to the mountain front, built by rivers sourced in the orogenic belt. Because of their great extent (compared with alluvial fans and floodplains), large areas of megafans remain unaffected for long periods (10 (super 3) -10 (super 5) y) by fluvial dynamics such as avulsion and meander migration. Morphogenetic stasis combined with stream behavior, suggests mechanisms of potential interest to aquatic evolutionary biology. A Avulsion. Rivers avulse into new radial positions, generating new alluvial tracts (or re-occupying old tracts) across megafan surfaces. This serves alternately to sever, and then to re-establish connections between trains of oxbow lakes and swamplands in a specific alluvial tract. Fans which straddle major divides play a special role: remotely-sensed evidence (Space Shuttle photos) of apex avulsion of the Parapeti R. suggests that this river can link aquatic fauna of the Amazon and Parana basins. Similar examples are known from other continents. B Aridity. Connections between megafan (and other) streams and foreland axial rivers can be severed periodically with the onset of aridity. Today the Parapeti, Pilcomayo, and Dulce Rivers all fail to reach axial rivers (Paraguay-Parana), becoming potential islands of speciation. Such islands achieve greater complexity when streams on neighboring megafans meet one another by avulsing simultaneously into the common depression between the fans. C Autochthonous stream network evolution. After trunk stream avulsion, streams sourced on megafans gradually become better integrated, as documented on Indian megafans. This implies the reconnection of at least some lake and swamp habitats isolated when the main stream avulsed away. D Stream invasion. Counter-intuitively, streams rising outside the confines of a megafan can establish new courses across the markedly flat topography of the megafan (as in northern India), thereby reconnecting isolated lakes and swamps.

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