Upstream and downstream controls of recent avulsions on the Taquari megafan, Pantanal, south‐western Brazil

Abstract

Avulsion, the natural relocation of a river, is a key process in the evolution of subaerial fans, river floodplains and deltas. The causes of avulsion are poorly understood, which is partly due to the scarcity of field studies of present avulsions. At present, two avulsions are occurring on the middle and lower Taquari megafan, Pantanal basin, south‐western Brazil. Here we present an analysis of the causes of these avulsions based on field and remote sensing data and show that avulsions on megafans can be controlled by both upstream and downstream processes. The middle fan avulsion (started in 1997–1998) is a result of upstream control: overbank aggradation was caused by the (variable) input of sandy sediment into the system, which caused channel‐belt superelevation and also created an easily erodible subsurface favouring bank retreat, crevassing, and scour of deep floodplain channels. The sandy subsurface in this area is inferred to have been a major factor in the causation of this avulsion under conditions of little gradient advantage. The lower fan avulsion (started c. 1990) results from interplay of upstream and downstream controls, the latter being related to the local base level (the Paraguay River floodplain) at the toe of the fan. Channel and overbank aggradation on the lower fan was influenced by fan sub‐lobe progradation and channel backfilling. Fan sub‐lobe progradation caused a significant gradient advantage of the avulsion channel over the parent channel. Avulsions are commonly supposed to be preferentially triggered by high‐magnitude floods, when there is considerable channel‐belt superelevation. However, both avulsions studied by us were triggered by small to average floods, with modest channel‐belt superelevation. We conclude that flood magnitude and channel‐belt superelevation have been overrated as causes of avulsion, and demonstrate additional causes that influence the growth of crevasses into avulsions. Copyright © 2012 John Wiley & Sons, Ltd.

Brazilian Pantanal: A Large Pristine Tropical Wetland

“…Large-scale avulsion is ongoing near the Caronal farm in the vicinity of the Taquari distributary lobe apex, as described in previous studies (Assine 2005;Assine et al 2005;Zani 2008;Buehler et al 2011;Makaske et al 2012;Zani et al 2012). The channel in this area has been subjected to rapid aggradation, resulting in the reduction of channel depth, emergence of sand bars, and triggering of avulsion processes with levee crevassing and formation of distributary channels (Fig.…”

Depositional development of the Muskeg Lake crevasse splay in the Cumberland Marshes (Canada)

“…Similar morphodynamics and splay development have also been observed on low‐gradient alluvial megafans (e.g. the Pantanal; Makaske et al ., ). Other splays described in literature often yield a typical dendritic pattern (examples in Bridge, ; Cahoon et al ., ).…”

Channel Avulsion Dynamics in Meandering Rivers

“…Increased discharge due to increasing precipitation in a region could affect sediment transport and channel morphology, thereby altering the likelihood of avulsion (Collischonn et al, 2001;Makaske et al, 2012). Increased flood frequency due to changed weather patterns and more-rapid snowmelt would increase the potential for trigger events.…”