Three-stage evolution of particle shape in headwater streams

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. This study unravels such complex interactions by evaluating the contributions of each process and revealing the factors that determine the rates of change in shape. We investigated changes in the roundness and circularity of basalt and shale particles in headwaters using the automated image analysis software Rgrains. The observed evolution of the roundness and circularity comprised three stages with different rates of increase. Both shape parameters initially increased rapidly, remained nearly constant, and then slowly increased downstream, 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. 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 particle durability, 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.