In the river bed of three particle sizes, fine sand is more difficult to withstand the erosion of the river, and it is easier to form a large range of erosion pits.
In the three grain-diameter riverbeds, fine sand is more difficult to resist the river's flushing, more likely to form a larger range of flushing pits.
However, in the riverbed with three particle sizes, the fine sand is more difficult to resist the river erosion, and it is easier to form a large range of scour pits.