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Gradual diversions of the Rio Pastaza in the Ecuadorian piedmont of the Andes from 1906 to 2008: role of tectonics, alluvial fan aggradation, and ENSO events |
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| Authors: | Carolina Bernal Frédéric Christophoul Jean-Claude Soula José Darrozes Luc Bourrel Alain Laraque José Burgos Séverine Bès de Berc Patrice Baby |
| Institution: | 1. UMR 5563 GET, Université de Toulouse-CNRS-IRD-OMP-CNES, 14 Avenue Edouard Belin, 31400, Toulouse, France 2. Petroamazonas, Av. Naciones Unidas E7-95 y Shyris, Quito, Ecuador, 4to piso, Quito, Ecuador 3. BRGM, Direction Régionale Réunion et Interrégionale Océan Indien, 5, Rue Sainte-Anne, BP 906, 97478, Saint-Denis Cedex, France |
| Abstract: | The successive courses of the Rio Pastaza in the upper Amazonian Puyo plateau (Ecuador) during the past century have been followed using historical maps, aerial photographs, satellite imagery, topographic and river long profiles, and field studies. The abrupt change in direction of the Rio Pastaza from transverse to longitudinal was a result of two avulsions occurred between 1906 and 1976 at the braided-meandering transition of the former alluvial plain. These avulsions are related to aggradation at the toe of a braided piedmont fan prograding on to a hinterland-dipping topographic slope formed by ongoing tectonic backtilting. The main avulsion proceeded by annexation of a south-dipping depression created in front of the cordillera by backtilting of the plateau. A partial and gradual avulsion having occurred upstream of the former site between 1976 and 2008 is marked by the progressive predominance of a newly formed inner branch. Tectonic backtilting enhanced aggradation upstream of the initial site while it offered the newly avulsed channel a still more favorable way along the cordillera by creating a westward lateral slope. The correlation between ENSO events and the occurrence of the 1976–2008 avulsions strongly suggests that the triggers of the avulsions were the floods caused by the high water and sediment discharges associated with ENSO (La Ni?a) events contrasting with the regular monthly discharge and the lack of actual ‘normal’ floods during the inter-ENSO periods. |
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