The maximum load of sediment that a stream can transport is called its capacity . Capacity is directly proportional to the discharge: the greater the amount of water flowing in the stream, the greater the amount of sediment it can carry.
The majority of a stream's sediment load is carried in solution (dissolved load) or in suspension. The remainder is called the bed load.
The suspended load is generally made up of lighter‐weight, finer‐grained particles such as silt and clay. Most of the sediment in a stream is carried as suspended load. It does not contribute greatly to stream erosion, since it is not in frictional contact with the stream bed. Bed load.
Traction occurs when these fragments move along by rolling and sliding. Turbulent or eddying currents can temporarily lift these larger grains into the overlying flow of water—the grains advance by short jumps or skips until the surge diminishes and then fall back to the bottom because of their greater weight.
A stream's competence is a measure of the largest‐sized particle it can transport; competence is directly proportional to a stream's velocity, which can vary seasonally. Because of increased capacity and competence, a single flood event can cause more erosion than a hundred years of standard flow. Previous Stream Erosion.
The saltation load is included in bedload because grain excursions into the fluid are short-lived. Saltating grains tend to travel much farther in air than their watery counterparts because the grain-to-fluid density contrast is much greater.
In this situation, clay particles tend to adhere to each other because of electrical charges inherent in their crystal structures.
As flow velocities increase, so too will the lift component of fluid forces and grains may temporarily leave the traction carpet, bouncing along with the flow; this process is called saltation. The saltation load is included in bedload because grain excursions into the fluid are short-lived. Saltating grains tend to travel much farther in air than their watery counterparts because the grain-to-fluid density contrast is much greater. Stand on any sandy beach on a blusterous day and you will witness first-hand the effects of grain saltation. Saltation produces many collisions, and not just against your bare legs. Saltation collisions also result in grain abrasion.
Both bedload and suspension load are important processes in the generation of sedimentary structures. In particular, bedload transport of loose sand is the critical process for growth of bedforms and their internal cross-stratification (crossbedding). The description of bedforms (crossbeds) and the flow conditions ( flow regime) under which they form have been described in other posts.
As the name suggests, this element of sediment movement consists of loose, granular particles at the sediment-water interface (such as a stream bed or tidal flat). Air or water that moves across the bed will being to move grains if the flow velocity is great enough to overcome the force of gravity and any resistance at grain contacts. This is the threshold velocity.
Here, fluid flowing over a sediment bed produces shear stresses that can be resolved into a component of drag (parallel to the bed) and a lift component normal to the bed. At the threshold velocity when the resultant fluid force on grains is greater than gravity, grains begin to roll, slide and jostle along the bed like a moving carpet – the traction carpet.
Saltation produces many collisions, and not just against your bare legs. Saltation collisions also result in grain abrasion. The short video shows a saltation load across a windy, black sand beach (sand consists of magnetite, ilmenite and pyroxenes).