THE ROLE OF SEDIMENT SUPPLY IN THE ADJUSTMENT OF CHANNEL SINUOSITY ACROSS TROPICAL RIVER BASINS

Meandering rivers are defined by their sinuous planforms and tendency to migrate. By lengthening the channel, meander migration clearly helps to determine the planform that arises. And yet, despite a range of studies highlighting sedimentological , biotic, valley-slope, and planform-curvature controls on the evolution of the meandering planform, there remains no statement on how channel sinuosity should adjust in response to meander migration. In short, we have yet to explain how quickly meandering rivers should lengthen as they migrate across their floodplains, one of the most fundamental aspects of the meandering process. Previous work carefully defined patterns by which the meandering planform can adjust, but the multitude of patterns suggested that the planform response to migration is controlled by too many variables to allow for a deterministic generalisation. Still, a formal evaluation of the nature of the relationship between lengthening and migration has been prevented by the absence of detailed observations across a range of meandering river settings. Here, we define the relationship for 30 meandering reaches of the Amazon Basin and Borneo using an archive of Landsat images. The rate that sinuosity adjusts with channel migration can be explained by the manner by which the meander planform is deformed: bends that extend normal to the downstream direction increase their sinuosity more slowly than bends that rotate downstream. Downstream rotating bends are more common within rivers with high sediment loads and appear to establish point bars with larger areal extents that expand throughout the meander. Upstream rotating meanders are associated with compound meander development or the deposition of upstream sediment lobes. Our results suggest that the evolution of freely meandering rivers is strongly tied to catchment-wide supplies of riverbed material. The turnover of floodplain sediments and habitats through meander migration is thus controlled by the full range of processes that ultimately produce and redistribute riverbed sediment.