Wash load is the portion of sediment that is carried by a fluid flow, usually in a river, such that it always remains close to the free surface (near the top of the flow in a river). It is in near-permanent suspension and is transported without deposition, essentially passing straight through the stream. It consists of the finest particles and can be defined by having a Rouse number of <0.8, meaning that the turbulent mixing velocity is far greater than the settling velocity.
The composition of wash load is distinct because it is almost entirely made up of grains that are only found in small quantities in the bed. Wash load grains tend to be very small (mostly clays & silts but some fine sands) and therefore have a small settling velocity, being kept in suspension by the flow turbulence.
Wash load is carried within the water column as part of the flow, and therefore moves with the mean flow velocity of main stream. Because there is little or no interaction with the bed, the particles extract only negligible momentum from the flow.
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