MINERALOGICAL TRACING OF THE ORIGIN OF SE DIMENTS AT RIVER CHANNEL CONFLUENCES

Mineralogical tracing of the origin of sediments at river channel conf luences. Sedimentary dynamics of river channel confluences may be stud ied through the use of natural tracers. The information supplied by tr acers is of crucial interest to understand the mixing processes of bea material from the confluent channels and of potential zones of accumu lation of contaminated sediments. We examined the bed material of a co nfluence where the mineralogy of the sediments of the tributary waters hed differs from that of the main channel. The potential of two tracer s (K feldspars and micas) and the heavy mineral concentrations is asse ssed. Due to its shape and density, K feldspars is the most efficient tracer to reveal the mixing between the sediment loads coming from the upstream channels. The spatial distribution of the tracer suggests a very rapid mixing of the loads : at a distance equivalent to 2.1 chann el width, mixing is nearly completed. Micas and heavy mineral concentr ations are less reliable indicators of the sedimentary dynamics. The t echnique was also applied at a confluence along.the St. Lawrence River . In this case, clear mineralogical differences between the sediments of the watersheds are absent. The spatial pattern of heavy mineral con centrations is controlled by the main morphological feature of the con fluence, that is, the avalanche face at the edge of a tributary mouth bar prograding within the St. Lawrence. Low heavy mineral concentratio ns are located on the avalanche face itself while higher concentration ms are found at the foot and top of the face This pattern reflects var iations in sorting regime rather than origin of particles.