The Grain-size Patchiness of Braided Gravel-Bed Streams – example of the Urumqi River (northeast Tian Shan, China)

摘要

In gravel-bed rivers, sediments are often sorted into patches of differentgrain-sizes, but in braided streams, the link between this sorting and thechannel morpho-sedimentary elements is still unclear. In this study, the sizeof the bed sediment in the shallow braided gravel-bed Urumqi River ischaracterized by surface-count and volumetric sampling methods. Threemorpho-sedimentary elements are identified in the active threads of theriver: chutes at flow constrictions, which pass downstream to anabranches andbars at flow expansions. The surface and surface-layer grain-sizedistributions of these three elements show that they correspond to only twokinds of grain-size patches: (1) coarse-grained chutes, coarser than the bulkriver bed, and (2) finer-grained anabranches and bars, consistent with thebulk river bed. In cross-section, the chute patches are composed of onecoarse-grained top layer, which can be interpreted as a local armour layeroverlying finer deposits. In contrast, the grain size of the bar-anabranchpatches is finer and much more homogeneous in depth than the chute patches.Those patches, which are features of lateral and vertical sorting associatedto the transport dynamics that build braided patterns, may be typical ofactive threads in shallow gravel-bed rivers and should be considered infuture works on sorting processes and their geomorphologic and stratigraphicresults.