Interrelationships of Channel Processes, Changes and Sediments in a Proglacial Braided River

Spatial and temporal variations in channel morphology, velocity and shear stress, bedload transport rate, and bedload and bed material size distribution were measured over five summer weeks in the braided Lyngsdalselva in Arctic Norway. Velocities and shear stresses reflected streamwise convergence and divergence of flow, and in turn influenced bedload transport rates although these depended also on the availability at the surface of appropriate-sized sediment. Large and small particles had almost equal mobility, so transport rate increased rapidly with shear stress as coarse sediment began to move and in so doing exposed finer particles. Size-selective transport is nevertheless indicated by down-reach and down-bar fining. The spatial patterns of shear stress and bedload transport rate within one divided-single-divided channel cycle changed with discharge, as did the resulting channel changes. In high meltwater flows, with a peak transport rate of 0.3 kg m-1 s-1, erosion was localised at two zones of flow convergence and tended to maintain the non-uniform channel geometry. During a higher rain-flood, with transport rates up to 3 kg m-1 s-1 a downstream fall in shear stress in the main distributary led to medial deposition and consequent lateral erosion, creating a wider, shallower, more uniform channel. Grain size distributions of bedload at high and low shear stresses match fairly well those of floodplain gravels and falling stage flood deposits respectively.