Flow recession as a driver of the morpho-texture of braided streams

Braiding has been widely studied in humid-temperate climates though rarely in arid environments. Morpho-texture of braided streams refers to the morphological-textural organization in a braid-cell (stream section including bars, anabranches and chutes) that may strongly relate to different processes and hydrological regimes. The objective of this study is to compare the morpho-texture of braided streams governed by diverse flow recessions in different climates. Measurements were conducted in the wadis Ze'elim and Rahaf, southeast hyper-arid Israel, in the Mediterranean Barranca-de-los-Pinos, central Spain and in humid-temperate braided systems, the La-Bléone River, Haute-Provence, France and in the Saisera and Cimoliana torrents, northeast Italy. Terrestrial laser scanning was used to produce point clouds and high resolution digital elevation models of the braid-cells. Wet braid-cells in humid-temperate environments were surveyed by a Total Station. Roughness and the upper tail of grain size distributions were derived from the scanned point clouds or from Wolman sampling. We found that anabranches are commonly finer-grained than the bars in dryland systems and in semi-arid sandy braided systems, contrary to the humid-temperate braided systems. In both climates, chutes are similar or coarser-grained than the bars which they dissect, in accordance with their steeper gradients due to the considerable bar-anabranch relief. The Saisera's morpho-texture is similar to that of the dryland braided channels, despite the very humid-temperate environment in which it is located, due to its short-lived, ephemeral type hydrograph. Hydrograph shape, specifically the duration of flow recession, typical of a climate but not confined to it, determines the morpho-texture of braided streams and the textural differentiation between a depositional bar and the adjacent anabranches. The morpho-texture of chutes and bars results also from local erosional processes affected by local topography, i.e. ungraded longitudinal profiles, and is not solely determined by flow recession. This new morpho-textural model enables identifying primary depositional and erosional braiding processes.