River Profiles and Fluvial Processes

Describe the long profile and changing cross profile of a river and its valley.
Explain fluvial processes including erosion, transportation, and deposition.
Describe the formation of erosional landforms such as interlocking spurs, waterfalls, and gorges.
Describe the formation of landforms resulting from erosion and deposition, such as meanders and ox-bow lakes.
Describe the formation of depositional landforms such as levées, flood plains, and estuaries.
Identify major landforms of erosion and deposition using an example of a river valley in the UK.
Analyze how physical and human factors affect flood risk.

A river changes along its journey

A river flows downhill from its source to its mouth. The source is usually in an upland area; the mouth is where the river enters the sea, a lake or another river. Its journey is divided into the upper, middle and lower courses.

The river channel is the trough containing the flowing water, bounded by its bed and banks. The river valley is the larger area of low land through which the river flows, including the valley floor and the slopes on either side. Both change downstream, but they are not the same feature.

River processes are called fluvial processes. Erosion wears away material, transportation moves it, and deposition lays it down. Together, these processes help explain why rivers and their valleys change shape.

The long profile: looking from source to mouth

A long profile is a side view along the length of a river. It shows how the river’s height and gradient change from source to mouth. Gradient means steepness: a large fall in height over a short distance gives a steep gradient.

A typical long profile is concave: it is steep near the source and becomes progressively gentler downstream. In the upper course, the profile can be uneven, with steep sections. The middle course has a gentler gradient, while the lower course is often almost flat.

This is a general pattern rather than an identical shape for every river. Individual rivers have variations along their profiles.

A typical river becomes less steep downstream, while its channel and valley become wider. The channel and valley are shown separately so their changes can be compared.

The cross profile: looking across the channel and valley

A cross profile is a slice across a river or valley, at right angles to the direction of flow. A channel cross profile shows the bed and banks; a valley cross profile extends across the surrounding land as well.

In the upper course, the river channel is usually narrow and shallow, with a rough bed and relatively large stones. The valley is narrow and typically V-shaped, with steep sides. Vertical erosion, which cuts downwards into the river bed, is dominant and deepens the channel and valley.

In the middle course, the channel becomes wider and deeper. The valley also widens, its sides become gentler, and a wider valley floor develops. Lateral erosion, which wears away the banks sideways, becomes more important and helps widen the valley.

In the lower course, the channel is generally widest and deepest. The valley has a broad, relatively flat floor with a wide flood plain: low-lying land beside the river that can be covered by floodwater. Deposition is important here, although erosion and transportation still occur.

Reading the cross profiles from upstream to downstream therefore reveals two linked changes: the channel becomes wider and deeper, while the valley becomes broader with gentler sides.

Why a gentler gradient does not always mean slower water

Although the gradient decreases downstream, average river velocity generally increases through much of the river’s course. Velocity means the speed of the flowing water.

In the upper course, a shallow channel and rough bed create considerable friction, resisting the flow. Downstream, the channel is generally wider and deeper, and its bed is smoother. Friction has less influence on the flow, so water can move faster despite the gentler gradient. However, the river may slow where it enters a lake or the sea.

This helps explain why a lower-course river can still transport sediment: a gentle gradient does not mean that the river has stopped moving material.

Four ways rivers erode

Hydraulic action is erosion by the force of the water itself. Flowing water strikes the bed and banks and breaks material away. The water provides the force; it does not need stones to scrape the rock.

Abrasion occurs when material carried by the river scrapes against the bed and banks. Stones act rather like sandpaper, wearing away the surfaces they touch.

Attrition occurs when stones carried by the river collide with one another. Pieces break off, so the stones become smaller, smoother and more rounded. This changes the river’s load rather than directly wearing away the banks.

Solution, also called corrosion, occurs when soluble rock dissolves into the water. The material is removed in dissolved form rather than as visible fragments.

These names describe how erosion happens. Vertical and lateral erosion describe its direction. For example, abrasion can wear down the bed through vertical erosion or wear away a bank through lateral erosion.

Four ways rivers transport material

A river’s load is the material it carries. Different parts of that load move in different ways.

Traction: large stones and boulders roll along the bed.
Saltation: smaller stones and pebbles bounce along the bed.
Suspension: fine particles, such as silt, are carried within the flowing water rather than moving along the bed.
Solution: dissolved material travels with the water. It is not a collection of visible particles.

A side-view river cutaway shows water flowing downstream above the bed. A large stone rolls along the bed by traction. Smaller pebbles follow short bouncing paths that repeatedly return to the bed by saltation. Fine sediment particles are carried within the water above the bed by suspension. Solution is shown by a callout pointing into particle-free water: dissolved material travels with the water and is not visible as stones or sediment particles. Sizes and movement paths are schematic, not to scale. — Coarse material moves along the bed, fine sediment can travel within the water, and dissolved material is carried in solution.

Imagine following a pebble downstream. It might bounce along the bed by saltation, collide with other pebbles and become smaller through attrition, then stop moving when the flow weakens. Transportation, erosion and deposition can therefore affect the same material at different times.

Solution connects erosion and transportation particularly closely: first the water dissolves soluble rock, then it carries the dissolved material downstream.

Why rivers deposit sediment

Deposition happens when a river no longer has enough energy to carry some of its load. The sediment settles or comes to rest.

Conditions that encourage deposition include:

Reduced discharge: less water passes along the river, for example after a period of low rainfall or when water is removed upstream.
A decrease in gradient: the river flows onto gentler ground and may lose energy.
Locally slower flow: water moves more slowly on the inside of a bend, allowing sediment to settle.
Entering a lake or the sea: the river flow slows as it enters a larger body of water.

As the flow loses energy, larger, heavier particles generally stop moving first. Finer sediment can remain in suspension and travel further. Dissolved material behaves differently from solid sediment and can be carried all the way to the sea.

Deposition is therefore selective: a river can deposit coarse material while continuing to transport finer particles. The balance between erosion, transportation and deposition changes both downstream and as the flow changes over time.

Profiles downstream

Long profile: generally concave; steep near the source, becoming gentler towards the mouth.
Channel cross profile: generally wider and deeper downstream.

Check whether the question asks about the river channel or the river valley. Channel width and depth do not describe the valley’s shape.

For a downstream comparison, use comparative wording: the valley becomes wider and its sides become gentler.

Distinguish abrasion, which wears away the bed and banks, from attrition, which wears down the material being carried.

Solution has two roles: dissolving rock is erosion; carrying the dissolved material is transportation.

When explaining deposition, link the change in conditions to reduced energy and the river’s inability to continue carrying its sediment.

Relating to rivers and the processes carried out by flowing river water.

The starting point of a river.

The place where a river enters the sea, a lake or another river.

A side view along a river from its source to its mouth, showing how its height and gradient change.

A cross-section taken across a river channel or its valley, at right angles to the direction of flow.

The steepness of a slope, expressed as the change in height over a horizontal distance.

The wearing away and removal of material by processes such as flowing water.

Erosion caused by the force of flowing water breaking material away from a river’s bed and banks.

Erosion caused by sediment carried by a river scraping and wearing away its bed and banks.

The wearing down of rocks carried by a river as they collide, becoming smaller and smoother.

As erosion, the dissolving of soluble rock into river water; as transportation, the carrying of dissolved material by the water.

Downward erosion of a river bed, deepening the channel and valley.

Sideways erosion of a river’s banks, widening the channel and valley.

The movement of material by a river.

Transportation in which large stones and boulders roll along the river bed.

Transportation in which small stones and pebbles bounce along the river bed.

Transportation in which fine sediment is carried within the flowing water.

The laying down of sediment when a river no longer has enough energy to carry it.

Sediment moved along a river bed, including material transported by traction and saltation.

The volume of water passing a point in a river each second, usually measured in cubic metres per second.