Linkage between sediment transport and supply in mountain rivers

In this chapter, the issue of sediment supply and availability and its impact on sediment transport in mountain rivers is discussed, with some emphasis on high-magnitude low-frequency flood events. A review of field observations and modelling studies show that the amount and grain size of sediment transported by a flood of a given magnitude depend on flow competence and transport capacity but also on the characteristics of the sediment available for transport. The latter vary in space and time due to the episodic nature of the processes associated with sediment supply to channels and to variations in the nature of the sources (e.g., glacigenic sediment, soil or landslide; lithology of source rock). Change in the nature of sources can be driven by tectonics and climate through landscape steepness, the impact of glaciation and deglaciation, precipitation and storm intensity and/or the stabilising effect of vegetation. In steep landscapes, boulders up to a few meters in size are commonly supplied to channels and can inhibit sediment transport and bedrock erosion when abundant. Extreme flood events are crucial in mobilising the whole grain population, including boulders that can be considered stable over centennial to millennial time scales. Extreme floods can have either a depositing or a cleaning effect: the former occurs when substantial amount of excess sediment is supplied simultaneously to the event, leading to net aggradation; in the latter, the flood may flush all sediment available out of a given reach, leading to bedrock exposure and a sediment flux limited by sediment availability. The nature of the sediment exported from a reach or basin during extreme events will also depend on the time between events and on the time needed for in–channel sediment replenishment. These observations stress the need for integrating the sources of sediment and the episodic processes that lead to sediment supply to channels and sediment transport in channels when modelling sediment dynamics or bedrock erosion in mountain catchments, or the development of stratigraphy in sedimentary basins.