River Processes: Erosion, Transport, and Deposition

River processes are the ways in which a river shapes its channel and valley: erosion wears away the bed and banks, transport carries the eroded material downstream, and deposition lays it down when the river loses energy. In brief: the four main types of erosion are hydraulic action, abrasion (corrasion), attrition and solution (corrosion). Material is transported as traction (rolled along the bed), saltation (bouncing), suspension (carried in the water) and solution (dissolved). A river deposits its load when its velocity or volume falls, such as at a bend, in a flood or at its mouth. The river’s energy depends on its velocity and discharge.

These processes explain the landforms along a river, from waterfalls to deltas. This guide describes each type of erosion, transport and deposition, the factors controlling them, compares them in a table, works through calculations of discharge and of the effect of velocity on the load carried, outlines the history of the study and corrects common mistakes.

How do rivers erode?

• Hydraulic action: the force of moving water pushes into cracks in the bed and banks and compresses the air in them; when the pressure is released, the rock is broken apart. It is strong in fast, turbulent flow and during floods.
• Abrasion (corrasion): the river’s load of sand and stones scrapes and grinds the bed and banks like sandpaper. It is the most effective process of erosion in most rivers and forms potholes.
• Attrition: the particles in the load collide with each other and break into smaller, rounder pieces; this makes the load smaller and more rounded downstream, but does not erode the channel directly.
• Solution (corrosion): soluble rocks such as limestone and chalk are dissolved by slightly acidic water.

Erosion may be vertical (cutting down into the bed, dominant in the upper course) or lateral (cutting sideways into the banks, dominant in the middle and lower courses). The rate is affected by the velocity, the amount and size of the load, the type of rock and the gradient.

How do rivers transport material?

• Traction: the heaviest boulders and cobbles roll along the bed.
• Saltation: pebbles and coarse sand bounce along the bed in short hops.
• Suspension: fine silt and clay are carried within the flow, which makes the river look muddy.
• Solution: dissolved minerals are carried invisibly in the water.

The river’s competence is the largest particle that it can move, and its capacity is the total load that it can carry. Both increase with velocity and discharge. The relation is steep: the mass of the largest particle that can be moved is roughly proportional to the sixth power of the velocity, so a small increase in speed in a flood can move far bigger rocks. The sediment moved by a river is its load, made up of the bed load (traction and saltation), suspended load and dissolved load.

When do rivers deposit?

Deposition happens when the river loses energy and can no longer carry its load. This happens when the velocity decreases: on the inside bend of a meander, where the flow is slow; when the river floods and spreads over the floodplain, so the shallow flow slows down; when it enters a lake or the sea; when the volume drops during a drought; and when the gradient decreases in the lower course. The largest particles are deposited first, and the finest last, so the sediment is sorted. Features formed include point bars, levees, floodplains and deltas. Human actions that influence these processes include dams (which trap sediment and cause erosion downstream), channel straightening and urbanization (which makes floods more rapid).

Processes compared

Worked example: discharge and carrying power

1. Discharge: the volume of water passing a point each second is Q = cross-section area × velocity. A channel 5 m wide and 1.2 m deep has a cross-section area of 5 × 1.2 = 6.0 m². At a mean velocity of 0.8 m/s: Q = 6.0 × 0.8 = 4.8 m³/s.
2. Flood: in a flood the depth rises to 2.0 m (the river is 5 m wide, in this example, as it is held by banks) and the velocity to 1.5 m/s: area = 5 × 2.0 = 10 m², and Q = 10 × 1.5 = 15 m³/s. This is 15 ÷ 4.8 = 3.1 times the normal discharge.
3. Competence: the velocity rose from 0.8 to 1.5 m/s, a factor of 1.5 ÷ 0.8 = 1.875. The mass of the largest movable particle increases roughly with the sixth power: 1.875⁶ = about 44, so the river could move particles about 44 times heavier than normal. (This is an approximate rule.)
4. Doubling of velocity: if the velocity doubles from 1 to 2 m/s, the competence increases by 2⁶ = 64 times.
5. Sediment volume per day: if the flood carries 2.0 kg of suspended sediment per cubic meter of water, then at 15 m³/s in one day (86,400 s) the water volume is 15 × 86,400 = 1,296,000 m³, and the sediment transported is 1,296,000 × 2.0 = 2,592,000 kg, which is about 2,600 tonnes.
6. Attrition: a pebble with a diameter of 80 mm in the upper course is rounded and reduced to 40 mm by the lower course, so its diameter halved; the size of the load decreases downstream.

How were river processes studied?

Grove Karl Gilbert, an American geologist, made pioneering experiments on sediment transport in flumes at the beginning of the 20th century. Filip Hjulström published the diagram in 1935 which relates the velocity of the flow to the erosion, transport and deposition of particles of different sizes, and it shows that fine clays need a higher velocity to be eroded than sand because of their cohesion. Luna Leopold and Thomas Maddock described how river channels adjust to discharge in the 1950s. Today, flumes, gauging stations and acoustic sensors give measurements of flow and sediment, and computer models predict erosion.

Which mistakes are common?

Mistake 1: Confusing abrasion with attrition

Abrasion wears away the bed and banks. Attrition wears down the particles of the load themselves.

Mistake 2: Thinking rivers erode only in the upper course

Lateral erosion is important in the middle and lower courses, where it creates meanders.

Mistake 3: Assuming a river deposits its load where it is fastest

Deposition occurs where velocity falls, such as the inside of a bend.

Mistake 4: Forgetting the dissolved load

Dissolved minerals are invisible but may make up a large part of the total load, especially in limestone areas.