DISCHARGE AND SEDIMENT SUPPLY CONTROLS ON EROSION AND DEPOSITION IN ADYNAMIC ALLUVIAL CHANNEL

Research on dynamic alluvial channels has recognised the influence on river channel change of both discharge and sediment supply, although i t has proved difficult to measure the latter. This paper presents the first accurate data from a dynamic alluvial channel mat describe the i nterrelated effect on channel morphological change of both discharge a nd sediment supply variations over different timescales. Reliable info rmation on the spatial patterns of erosion and deposition were obtaine d using combined photogrammetry and tacheometric survey of the meltwat er stream of a glacierised catchment over a 5 week period. Over the fu ll time-period, the morphological evidence suggested the passage of a wave of sediment. Counter-intuitively, the period of aggradation was a ssociated with increasing diurnal peak and base flow discharge magnitu de, while degradation occurred during a period of steady peak diurnal discharge. This suggests that the channel change was controlled by var iation in upstream sediment supply. Over a shorter timescale, informat ion on diurnal changes in morphology reveals that although discharge w as responsible for some aspects of channel change, this was modified b y the effects of upstream sediment supply. Consideration of the spatia l patterns of erosion and deposition at the within-reach scale reinfor ces this point, with spatial aspects of morphological change being dri ven by discharge and sediment supply fluctuations, but modified by spa tial feedbacks associated with internal channel morphology. The implic ations of these findings for fluvial geomorphology in general are cons idered.