Wash load is similar to a

suspended load The suspended load of a flow of fluid, such as a river, is the portion of its sediment uplifted by the fluid's flow in the process of sediment transportation. It is kept suspended by the fluid's turbulence. The suspended load generally consists ...

, but wash load sediment never interacts with the

bed load The term bed load or bedload describes particles in a flowing fluid (usually water) that are transported along the stream bed. Bed load is complementary to suspended load and wash load. Bed load moves by rolling, sliding, and/or Saltation (geolo ...

. All of the sediment in the wash load stays suspended in the water throughout the channel (this concept has been debated). Wash load refers to a

river A river is a natural stream of fresh water that flows on land or inside Subterranean river, caves towards another body of water at a lower elevation, such as an ocean, lake, or another river. A river may run dry before reaching the end of ...

's ability to move

sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...

through a channel. Hans Albert Einstein described wash load as "if the sediment is added to the upstream end of a concrete channel and the channel is swept clean, and the sediment has not left any trace in the channel". The sediments in the wash load are generally smaller than .0625 mm. What determines the wash load is the relationship between the size of the

and the size of the particles that never settle in the "fine sediment load" or wash load.

Composition

Composition is a good way to classify the difference between wash and bed loads because the definition is distinct. The wash load is almost entirely made up of grains that are not found in quantity in the bed. Wash load grains tend to be very small (mostly

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, ). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impuriti ...

and

silt Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension (chemistry), suspension with water. Silt usually ...

s, but also some fine

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is usually defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural ...

s) and therefore have a small

settling Settling is the process by which particulates move towards the bottom of a liquid and form a sediment. Particles that experience a force, either due to gravity or due to Centrifuge, centrifugal motion will tend to move in a uniform manner in the ...

velocity, being kept in suspension by the flow

turbulence In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to laminar flow, which occurs when a fluid flows in parallel layers with no disruption between ...

. The composition of the wash load changes as the bed load changes. According to Einstein, all particles that are not significantly represented in the deposits of the bed "must be" considered wash load. This means that as the bed load moves down the river and the size of the bed load is reduced so is the size of the wash load. This is shown by Einstein's "suggestion that the largest sediment size may be arbitrarily chosen as the grain diameter of the D₁₀ (wash load) of which 10% of the bed sediment is finer.” Wash load also has a different composition than the bed load. The bed loads parent material is from directly around the water feature, but the wash loads source of sediment is defined as "sediment in transport that is derived from sources other than the bed."

Sediment motion

Sediment motion is one way of classifying part of either the bed or wash load. Wash load particles do not experience the motions that are typical of the bed load because they are suspended in water.

"Bed load is made up of particles that are rolling, sliding or saltating and which are, therefore, in either continuous or intermittent contact with the bed. Suspended sediment moves in the water column above the bed and is rarely in contact with the bed".

Velocity

Wash load is carried within the water column as part of the flow, and therefore moves with the mean flow velocity of mainstream. Because there is little or no interaction with the bed, the particles extract only negligible momentum from the flow.

Sampling techniques

Lowering and raising a "nozzle" sampler through the water column collects wash load or sediment load data. This technique fails to sample the sediment load that is close to the bed, but sediment load close to the bed can be estimated with the Einstein-Brown transport formula. There is also another device the Helley-Smith sampler used by the

United States Geological Survey The United States Geological Survey (USGS), founded as the Geological Survey, is an agency of the U.S. Department of the Interior whose work spans the disciplines of biology, geography, geology, and hydrology. The agency was founded on Mar ...

, which can sample the unmeasured sediment load close to the bed. There are also equations that can be used to estimate wash load.

References

{{Rivers, streams and springs Sedimentology Sediments Geomorphology