Shape evolution of bulk sediment in headwater streams: effects of rock type and particle size

Abstract. Given the importance of sediments in fluvial morphodynamics, studying how sediment particle shapes change during mass loss is important for understanding the morphology and change rates of fluvial landscapes. Particles of riverbed materials tend to become more rounded and circular downstream, but this trend can often be obscured because it results from processes that increase or decrease shape parameters to various degrees. Relative importance of the relevant processes, such as chipping, lateral sediment supply, and the production of finer particles during mass loss, may differ depending on rock type, lateral sediment supply, and particle size. This study evaluates the contributions of each process and reveals the factors that determine the rates of change in shape parameters of riverbed material. We investigated changes in the roundness and circularity of basalt and shale particles in headwaters using the automated image analysis software Rgrains. Roundness is calculated using curvature of particle edges, while circularity is calculated using the entire perimeter of a particle. The observed pattern of downstream evolution of the roundness and circularity was clearly different between the upstream and downstream part of the studied area. Both shape parameters initially increased rapidly and remained nearly constant, indicating that the dominant process affecting the particle shape changed during a few kilometers of transport. These punctuated shape changes result from the hillslope sediment supply and the addition of rock fragments produced by chipping and fragmentation, of which finer fragments were found to significantly alter the downstream evolution of the shape. This interpretation is supported by the fact that roundness of basalt particles slowly increased in the downstream part that lacks supply of basalt particles from hillslopes. The rate of increase in the roundness and circularity of the particle shape depended on the rock type and grain size. The rates for the shale particles were higher than those for the basaltic particles. Grain size clearly affected the shape change rates of basalt particles but not of shale particles. We interpreted these differences between rock type and grain size to be associated with material strength, weathering mechanisms and speed, and total residence time in the channel. These findings demonstrate that image-based measurements of shape parameters in headwaters enables a detailed examination of the mechanism and rates of changes in particle shape.