Discharge and sediment supply controls on erosion and deposition in a dynamic alluvial channel

Research on dynamic alluvial channels has recognised the influence on river channel change of both discharge and sediment supply, although it has proved difficult to measure the latter. This paper presents the first accurate data from a dynamic alluvial channel mat describe the interrelated effect on channel morphological change of both discharge and sediment supply variations over different timescales. Reliable information on the spatial patterns of erosion and deposition were obtained using combined photogrammetry and tacheometric survey of the meltwater stream of a glacierised catchment over a 5 week period. Over the full time-period, the morphological evidence suggested the passage of a wave of sediment. Counter-intuitively, the period of aggradation was associated with increasing diurnal peak and base flow discharge magnitude, while degradation occurred during a period of steady peak diurnal discharge. This suggests that the channel change was controlled by variation in upstream sediment supply. Over a shorter timescale, information on diurnal changes in morphology reveals that although discharge was responsible for some aspects of channel change, this was modified by the effects of upstream sediment supply. Consideration of the spatial patterns of erosion and deposition at the within-reach scale reinforces this point, with spatial aspects of morphological change being driven by discharge and sediment supply fluctuations, but modified by spatial feedbacks associated with internal channel morphology. The implications of these findings for fluvial geomorphology in general are considered.