Controlling factors of downstream-fining trend in gravel-bed rivers: examples from western and northern Iran

The downstream-fining trend in river-bed sediments occurs due to the hydraulic sorting and grain erosion. Downward decrease in velocity and stream power causes selective sediment entrainment which results in a fining trend in the grain size. However, some discontinuities occur in this trend. In this study, five controlling factors which cause these discontinuities were tested in the studied rivers: side sediment sources, change in the basin lithology, mining, fault and basin slope. The first factor causes two types of discontinuities: 1- abrupt increase in the grain size (due to confluence of a gravel-bed side channel) and 2- a temporary coarsening trend (in case of side-channel screes). Mining (i.e. river sand and granule) results a temporary increase in the fining trend and occurrence of fault (causes abrupt increase in the grain size). Change in the basin lithology also causes abrupt increase in the grain size (if the downstream rocks are less resistant to erosion).