Geometry and Grain Size Characteristics of the Basal Surface of a Braided River Deposit

Description of the three-dimensional geometry and grain-size patterns of fluvial deposits and their relationship to the morphodynamics of the river has been an elusive objective in sedimentology, hindered by limited exposure and the time scale of fluvial dynamics. In small-scale physical models the time scales are short enough to map river morphology and sedimentology during significant (and continuous) morphological development of the river and its deposits, especially for braided rivers. Using close-range digital photogrammetry, the dynamics of the river morphology, and resulting deposit geometry and sedimentology, were captured using digital elevation model differencing techniques, combined with automated grain-size mapping from image texture analysis. Using these novel methods we show the temporal development and characteristics of braided river deposit geometry, and, for the first time, map the characteristics and development of the basal surface of braided river deposits in relation to river morphodynamics and formative processes. The basal surface has considerable relief, wide variation in grain size similar to that of the river as a whole, develops progressively over time by switching of channels producing adjacent patches and ribbons of basal incision of different ages, and is only partially related to bed scour associated with laterally migrating channel confluences