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

Correct — D, Traction load. A river carries its load in four ways, and the division is by particle size and by how much energy the flow must spend on each. The heaviest fragments — cobbles and boulders — are never lifted clear of the bed at all; they are pushed, rolled and dragged along it, and that dragging is traction. Together with saltation it makes up what geologists call the bed load, as against the suspended and dissolved fractions that travel within the water itself.

• (a)Solution load — The dissolved fraction — calcium, magnesium and other salts carried invisibly in the water. It has no particle size at all, and is largest in limestone country.
• (b)Suspension load — Silt and clay held up within the flow by turbulence. This is what colours a river brown in flood and what builds a delta, but the particles are fine, not the heaviest.
• (c)Saltation load — Sand and small gravel that hop or bounce along the bed in short leaps, lifted briefly and dropped again. Close to the answer, and it shares the bed with traction, but the leaping fraction is lighter than the fraction that only rolls.

A river's capacity to move material rises very steeply with velocity, which is why a stream that carries only silt in the dry season can roll boulders in flood. The four transport modes sit along a size gradient: dissolved matter, then fine particles in suspension, then sand bouncing in saltation, then the coarsest fragments rolling in traction. Deposition reverses the order as the flow slows, which is why gravel is dropped first and clay last.

The stem names the discriminating detail twice over — heaviest, and rolled along the river bed. Saltation is the intended trap because it also happens at the bed, and the distinction between hopping and rolling is the only thing separating the two options. Solution and suspension can be dismissed at once, since neither involves contact with the bed.

• Traction is the rolling and dragging of the coarsest fragments along the channel floor.
• Saltation is the bouncing of sand and small gravel in short hops.
• Traction and saltation together form the bed load; suspension and solution travel within the water.
• Suspended load is usually the largest fraction by weight in a large Indian river during the monsoon.
• Attrition, abrasion, hydraulic action and solution are the four erosional processes that generate this load.

• Choosing saltation because it also happens at the bed; the leaping fraction is the lighter one.
• Treating suspended load as the heaviest because it is the most visible.

A one-term definition item where the stem's own adjectives point at the answer.

No directly related past PYQ was found.

• practice — not a real PYQ

  Which two transport modes together make up a river's bed load?

  • (a)Solution and suspension
  • (b)Saltation and traction
  • (c)Suspension and saltation
  • (d)Solution and traction

  Answer(b) Saltation and traction — both involve contact with the channel floor, whereas solution and suspension travel within the water.

• practice — not a real PYQ

  The wearing down of load particles against one another as they are carried along is called

  • (a)Abrasion
  • (b)Attrition
  • (c)Hydraulic action
  • (d)Corrosion

  Answer(b) Attrition — the fragments collide and grind each other smaller and rounder as they move.