All of the sediment in the wash load stays suspended in the water throughout the channel[1] (this concept has been debated).
[2] Wash load refers to a river's ability to move sediment through a channel.
Composition is a good way to classify the difference between wash and bed loads because the definition is distinct.
According to Einstein, all particles that are not significantly represented in the deposits of the bed "must be" considered wash load.
There is also another device the Helley-Smith sampler used by the United States Geological Survey, which can sample the unmeasured sediment load close to the bed.