CONTROL OF RIVER MORPHOLOGY BY THE GRAIN-SIZE OF SEDIMENT SUPPLIED

The assumption that the morphology and grain-size distribution of river sediments are largely controlled by water flow variables is questioned. Water flow variables (such as mean velocity, shear velocity or stream power) tend only to predict correctly the amount of sediment transported if the flow is sediment-saturated or if the sediment load is transport-limited. In natural river systems, flows tend to be sediment-undersaturated, and the sediment load is supply-limited. Under natural conditions, the sediment supply will depend on episodes or ''sediment moves'' that may involve hillslopes, banks, tributaries, or the sediment bed, in addition to transfer from the immediately upstream channel reach. Both spatial and temporal changes of one of these types of sediment supply produce changes of river morphology, at least locally, and over historical orders of time. But the same processes are likely to be critical in larger, basin-scale, river systems, and over geological time periods.