Competence id the maximum size (calibre) of load a river is capable of transporting whereas capacity refers to the total volume of sediment a river can transport. At low velocity only fine particles may be transported (clays, silt and fine sands). Large-calibre material can be moved when velocity increases. Because the maximum particle mass that can be moved increases with the sixth power of velocity, when discharge levels are high, for example during a flood, much larger boulders can be moved.
The Hjulstrom curve illustrates the relationship between velocity and competence. It shows the velocities at which sediment will normally be eroded, transported or deposited. The mean or critical erosion velocity (also known as entertainment velocity) shows the velocity required to pick up and transport sediments of varying calibre. The mean fall or settling velocity curve indicates the velocity at which each calibre of sediment is deposited because the energy level is too low to keep it in suspension.Two key points may be made: * Very fine particles (see 2 below) may require higher velocities before they erode than sand (see 1 below) because of the cohesive properties of the fine clays and silts.
* The velocity required to keep fine particles in suspension is less than the velocity required to erode them. Therefore fine, platy-clay particles can stay in suspension even when water is almost still (see 4 below) whereas only a very small drop in velocity can mean that sediment of sand to boulder calibre will be deposited (see 5 below).