Observations of bedload movement, bar development and sediment supply in the braided Ohau river

Abstract Relative bedload transport rate and hydraulic parameters were measured on two occasions in a reach of the braided, gravel‐bedded Ohau River. Each reach contained a deep, fast‐flowing chute leading to an area of diverging, shallow flow which contained a middle bar. The measurements are self‐consistent, and indicate that, where flow is concentrated in a deep chute, shear stress is high, but where flow diverges, depth, slope and shear stress decrease. In the first survey, the bed was scoured in the chute and sediment transport rates were high, but where flow diverged sediment transport rate decreased. It appears that deposition leads to bar growth, bar growth in turn reduces slope and depth, deposition is encouraged, and the bar continues to grow vertically, laterally and upstream. In the second survey no sediment transport was observed, despite hydraulic conditions very similar to the first survey. The absence of sediment transport is attributed to the cessation of sediment supply to the river channel.