Wanderlust World Wanderlust World The amount and size of sediment a river can transport—known as its capacity and competence—depend on a dynamic interplay of hydraulic forces and geological supply. These factors determine everything from the shape of the river channel to the formation of deltas and floodplains.
Higher discharge increases flow velocity and turbulence, which keeps particles suspended and moves larger bedload material. Flood events are responsible for the vast majority of a river's annual sediment transport.
A river cannot transport sediment that isn't available. Upstream geology, tributary inputs, and hillslope processes control the sediment supply. A sediment-starved river below a dam, for example, will have high transport capacity but low actual load.
Coarse particles (gravel, cobbles, boulders) require significantly more energy to move than sand, silt, or clay. The competence of a river is defined as the maximum particle size it can transport at a given flow, while capacity refers to the total sediment load (mass per unit time).
These values represent the upper limit of bedload transport during high-flow events. Suspended loads of fine silt and clay can be orders of magnitude greater in mass.
Rivers also carry a substantial dissolved load of salts and minerals derived from chemical weathering in the basin. This invisible cargo is influenced by bedrock geology, climate, and residence time, and it plays a critical role in ocean chemistry and global biogeochemical cycles.