Crevassing and capture by floodplain drains as a cause of partial avulsion and anastomosis (lower Rio Pastaza, Peru)

Avulsion is the main process at the origin of anastomosing rivers. This study illustrates 3 examples of avulsions resulting from crevasse splays evolving in anastomosed channels along the Rio Pastaza, a tropical humid river sourced in the Ecuadorian Andean Cordillera and flowing into the Amazonian foreland. The Lower Pastaza flows in an alluvial plain, with no tectonic influence and an average monthly rainfall equally distributed throughout the year. Based on the analysis of satellite image recorded over the period 1977-2008, three cases have been studied. The first one began in 1990 with crevassing of natural levees of the right bank of the Pastaza main channel and the formation of a small channel linking up with a pre-existing tributary to this main channel. A splay formed at the confluence beheaded the tributary which became an anabranch of the main river. Downstream, two other avulsions developed from crevasse splays on a low gradient floodplain. In both cases, capture of one of the distributary channels flowing on the splay by a pre-existing drain of the floodplain and consecutive headward erosion arrives to disconnect the other drains and capture their flow into a single-thread channel. As this channel rejoins the Pastaza main channel downstream, this process gives rise to the larger-scale anastomosing system which characterizes the lower reach of the Rio Pastaza.