The load carried through the mechanism of mechanical transportation are grouped in to two categories as Suspended load and Bed load.
Suspended Load
Fine particles of clay and silt are transported in suspension. These particles sometime float on the surface of the water and sometime they are carried within the water and become part of the fluid mass.
The intensity of turbulance and the velocity of the river-flow determines the length of time a particle remains in suspension.
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Bed Load
It comprises the heavier particles of sand, pebbles, gravels and cobbles which move close to the channel floor by rolling or sliding and an occasional low leap. It is moved along on, and supported by the bed of the channel.
The transportation of the bed-load takes place in two ways:
(i) Saltation and
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(ii) Traction.
(i) Saltation
Here a particle resting on the river-bed is temporarily lifted up by the eddies and is carried to some distance before it again falls to rest.
The smaller the particle, the higher the lift and the longer the jump. This process of saltation is mostly effective in the transportation of sand grains.
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(ii) Traction
The transportation of sediments by creeping is known as ‘traction’. Bigger fragments of rocks like boulders, cobbles and pebbles are rolled along the bottom or they may slip and slide downstream.
The weight of fragments that can be rolled along the bottom is proportional to the sixth power of steam velocity, according to the Airy’s law. Thus a mountain river transports rock-fragments of bigger size much more in comparison to that of the flat-country rivers.