The heaviest rocks, which are rolled along the river bed in river transport, come under which type of load?

In fluvial geomorphology, the transport of sediment occurs through four primary mechanisms: solution, suspension, saltation, and traction. Traction load refers to the largest and heaviest rock fragments, such as boulders and large pebbles, which are too heavy to be lifted by the river's current [2]. Instead, these materials are rolled or dragged along the river bed by the force of moving water. This process is distinct from saltation, where intermediate-sized particles bounce or jump along the bed. Traction is most common in the upper reaches of a river where the gradient is steep and the energy is sufficient to move large debris, though the velocity must be high to overcome the friction of the river bed. Collectively, materials moved by traction and saltation are referred to as the bed load [2].

Sources

1. [2] https://www.austintexas.gov/sites/default/files/files/Water/CER/river_process_may_2013s.pdf