Spatial and Temporal Evolution of an Avulsion on the Taquari River Distributive Fluvial System from Satellite Image Analysis

Abstract

Avulsion processes have been observed and analyzed using field based studies, however, they have rarely been analyzed through use of satellite imagery even though such an approach allows documentation of spatial and temporal variations of the resulting deposits. In order to document changes that are occurring at the Caronal avulsion on the Taquari River, Brazil, a time series of LANDSAT imagery from 1985 to present (2010) was compiled. As of July 2010, the imagery shows that the avulsion is not yet complete, with discharge being routed to both the parent channel and the avulsion channel in the floodbasin. The avulsion was initiated as flow was diverted from the parent channel to the floodbasin through two small crevasses, allowing the development and evolution of splays, with a single splay stabilizing only since 1999. The avulsion area is marked by the growth of new splays, the formation of an avulsion belt, and progradation of avulsion deposits into the floodbasin. The parent channel downstream of the avulsion site has experienced decreasing width through time, associated with the higher volumes of water and sediment being captured by the avulsion channel. Because this avulsion is not yet complete, we expect more morphological changes to occur as the avulsion belt evolves into the primary channel. Rapid sedimentation caused by this avulsion and the 24-year time series of imagery that captured this change allowed us to construct a 3D block representing the evolution of depositional environments associated with avulsion, using time in the vertical dimension. Thus we could represent the evolution of geomorphological patterns, including floodbasin, proximal levee, distal levee, and channel deposits, and project their distributions through time. This block of evolving sedimentary settings can be used as a guide for future sedimentologic studies by offering visual clues regarding the way which depositional systems track avulsion-initiated changes.