Sedimentology of Large Rivers

The logical corollary from the above is that if key processes, such as unit bar development, are different in large rivers, then this will result in significant differences in their depositional sedimentology when compared to their small river counterparts. However, there are also few descriptions of the alluvial architecture of large braided rivers in the literature with which to test such an idea. Critically, the description of ancient deposits from large rivers is often limited by: (i) short and incomplete outcrop and the lack of exposures for large rivers; and (ii) the fact that interpretations of outcrop often involve a priori assumptions concerning the influence of some controlling variables. There is thus presently no quantitative facies model for large braided rivers and we have little confidence in our ability to reconstruct the dimensions of bedforms and channel geometry from the preserved deposits of large rivers. Although recent work has suggested there may be a degree of scale invariance in braid-bar depositional facies, this idea has yet to be tested across a wide range of river sizes and controlling boundary conditions. Additionally, no current facies model documents how suspended sediment may influence within-channel bedforms, braid-bar morphology, sedimentation and the preserved alluvial architecture, despite the fact that many large rivers possess such high suspended sediment loads. There is thus a clear need to quantify the facies distributions and alluvial architecture of the largest braid-bars to establish how they correspond with current models and to determine what influence high suspended sediment loads may have on the preserved deposits.