In hydrology stream competency, also known as stream competence, is a measure of the maximum size of particles a stream can transport.[1] The particles are made up of grain sizes ranging from large to small and include boulders, rocks, pebbles, sand, silt, and clay. These particles make up the bed load of the stream. Stream competence was originally simplified by the “sixth-power-law,” which states the mass of a particle that can be moved is proportional to the velocity of the river raised to the sixth power. This refers to the stream bed velocity which is difficult to measure or estimate due to the many factors that cause slight variances in stream velocities.[2]
Stream capacity, while linked to stream competency through velocity, is the total quantity of sediment a stream can carry. Total quantity includes dissolved, suspended, saltation and bed loads.[3]
The movement of sediment is called sediment transport. Initiation of motion involves mass, force, friction and stress. Gravity and friction are the two primary forces in play as water flows through a channel. Gravity acts upon water to move it down slope. Friction exerted on the water by the bed and banks of the channel works to slow the movement of the water. When the force of gravity is equal and opposite to the force of friction the water flows through the channel at a constant velocity. When the force of gravity is greater than the force of friction the water accelerates.[4]
This sediment transport sorts grain sizes based on the velocity. As stream competence increases, the D50 (median grain size) of the stream also increases and can be used to estimate the magnitude of flow which would begin particle transport.[5] Stream competence tends to decrease in the downstream direction,[6] meaning the D50 will increase from mouth to head of the stream.
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