River Processes

Examples of river processes

Now that you know how to define fluvial or river processes, it is time to learn examples of them. Three examples of fluvial processes are:

1. Erosion
2. Transportation
3. Deposition

Let's look at these in more detail.

River erosion processes

River erosion is the process through which the river bed and bank are worn away. It also includes the wearing down of the sediments carried by the river. River erosion can be lateral and vertical.

Lateral erosion occurs when the river erodes its banks, causing its channel to widen. It more commonly occurs in the middle and lower courses of the river.

Vertical erosion occurs when the river erodes its bed. This causes the deepening of the bed and commonly occurs in the river's upper course. Look at figure 1 to see how lateral and vertical erosion work. To find out more information about the upper, middle and lower courses of a river, read our explanation on River Landscapes.

Types of river erosion processes

So now you know that rivers erode both vertically and laterally. But how do they do this? There are four types of river erosion processes, which we shall discuss in the following sections. They are:

1. Hydraulic action
2. Abrasion
3. Attrition
4. Solution

Hydraulic Action

Hydraulic action is the removal of material by the force of the water itself. When fast-flowing water hits the river bed and banks, it enters into any cracks which may be present and causes the air within the crack to compress. When the water flows out, the air in the crack expands. The continuous expansion and compression of the air by the water widens the cracks and causes pieces to break off.

Abrasion

Abrasion is the wearing away of material by sediments carried by the river. Sediments such as sand, pebbles, or boulders scrape or grind along the river bed and banks and cause pieces to break off.

Attrition

You may have noticed that river stones are often round and smooth. This is caused by attrition. Attrition affects the river's load (material carried by the river). As the rocks and pebbles are carried downstream, they hit each other and the river bed. This causes them to gradually become smoother and smaller.

Solution

Solution or corrosion is when rivers dissolve away rocks such as limestone or chalk due to the mildly acidic effect of the carbon dioxide dissolved in the water, forming a weak carbonic acid.

River transportation processes

Transportation is when rivers carry their load downstream.

Types of river transportation processes

Like erosion, there are four types of river transportation processes:

1. Traction,
2. Saltation
3. Suspension
4. Solution

Traction

Traction is when large rocks or boulders, which form part of the river's bedload, are rolled or pushed along the river bed by the force of the water. This requires the most energy. Therefore traction occurs intermittently.

Saltation

Saltation occurs when pebbles or stones are bounced or hopped along the river bed in an almost "leapfrog" type movement.

Suspension

In suspension, fine particles such as silt or clay are held up and carried within the flow of the water itself. The currents of the moving water support the particles.

Solution

In solution, materials from soluble rocks such as chalk and limestone dissolve in and are carried by the water. This process requires the least energy.

River deposition

Deposition is the process through which the river drops its load. This happens when the river's speed decreases due to a loss of energy or a reduction in discharge. Deposition occurs when the river channel widens, and the bed becomes shallower, the gradient of the channel decreases and when the river enters a lake or an ocean. The latter is why there are always deposits at the mouths of rivers.

Fig. 4 - Deposits at the mouth of the Amazon River, Brazil

In most rivers, deposition takes place in a sequence according to the size of the material. Larger, heavier materials such as boulders are deposited first, and finer materials such as clay are deposited last.

River processes and landforms

The river or fluvial processes of erosion and deposition, aided by transportation, help the river to alter the landscape and form different types of fluvial landforms.

Landforms from river erosion

The river's upper course is dominated by vertical erosion since most of the river's energy is used to cut downward. In this part of the river, you will find the fluvial landforms of waterfalls, gorges and interlocking spurs.

Waterfalls and gorges

Waterfalls are sudden drops along a river's course. They form when the river erodes exposed soft rock which is layered over a band of hard rock. Over time the hard rock is left overhanging. The overhang will then collapse because it is no longer supported. After the collapse, the stream falls over the remaining ledge as a waterfall. When waterfalls continuously erode backwards, they leave a steep-sided valley called a gorge. The image below shows a waterfall and a gorge.

Fig. 5 - Corrieshalloch Gorge waterfall, Scotland

Interlocking Spurs

Interlocking spurs are outcrops of harder rock that alternate projecting from either side of a v-shaped valley which causes the river to flow around them. They are also found in the river's upper course and fit together in a "zip-like" manner.

Fig. 6 - Interlocking spurs along a tributary of the River Severn, UK

Landforms from river erosion and deposition

In the river's middle course, erosion and deposition work together to form landforms. Here, vertical erosion replaces lateral erosion. In this part of the river, you will find the fluvial landforms of meanders and ox-bow lakes.

Meanders

Meanders are bends in the river. They develop when lateral erosion, which occurs on the river's outer bank, is coupled with deposition on the river's inner bank. These dual processes working together eventually cause the river to curve.

Fig. 7 - Meanders in Kale Water from Woden Law, Scotland

Ox-bow lakes

An ox-bow lake is a horseshoe, or crescent-shaped lake found close to meanders. They represent former meanders that were cut off by the river during a period of flooding.

Fig. 8 - The presence of ox-bow lakes along the River Clyde-UK

Landforms from river deposition

In the river's lower course, deposition is the dominant process because the river's gradient is very gentle, and the speed of the water is also slower. Here, you will find the fluvial landforms of flood plains, levees, and estuaries.

Flood plains

A flood plain is the area of flat land on either side of a river. They are commonly made of silt, fertile, and often flooded by the river.

Levee

A levee is a raised river bank. Continuous deposition of sediments along the river bank after repeated flooding events causes their formation.

Fig. 9 - Levee along the Sacramento River, US

Estuaries

An estuary is where a river meets the sea and becomes tidal. Estuaries represent the transition from the fluvial zone to the marine zone.

Fig. 10 - Exe estuary, UK