Fill characteristics of abandoned channels and resulting stratigraphy of a mobile sand‐bed river floodplain

Floodplains are diverse sedimentary environments where infill processes of abandoned channels interact with overbank sedimentation and bank erosion. The result, particularly in river systems with high suspended load and rapid channel migration, is a complex three‐dimensional mosaic of deposits with spatial variability in terms of grain‐size, age, organic carbon content and resistance to erosion. Abandoned channels represent a significant deposition volume in fluvial systems that can accommodate large proportions of the equivalent material mobilised during their abandonment. However, time scales and fill processes vary between different kinds of abandoned channels and the sediment calibre involved and are not fully understood, particularly in respect to highly dynamic sand‐bed rivers. This study investigates time scales and spatio‐temporal patterns of infill of abandoned chute channels and abandoned channel segments left behind following neck cutoff of meander bends. The study focuses on the Rio Beni, a large, tropical, sand‐bed river in the Bolivian Amazon basin. Electrical resistivity ground imaging is used to elucidate the stratigraphy of floodplains and satellite imagery is employed to investigate contemporary fill processes and rates. Given suitable bend migration patterns, chute channels may remain stable for several years but are eventually abandoned and rapidly filled with bed material during a single flood season. Smaller scroll sloughs can convey coarse bedload across point bars and, when filled, present stratigraphic bodies similar to chute fills. Abandoned meander bends tend to develop plug bars at both ends immediately after cutoff. Of these bars, the downstream plug aggrades at a faster rate due to the often larger diversion angles with the main channel and efficiently seals off the bend. The subsequent infill of the channel is a function of hydraulic connectivity and distance to the active channel as well as rate of lake deposition. Considerable overbank deposition can increase the spatial sedimentological heterogeneity of these floodplains, which needs to be taken into account in floodplain evolution models.