River Erosion Landforms

Erosional landforms

The process of erosion forms different landforms. They are usually found in the upper and middle course of the river. Let's look at some of the landforms associated with erosional landforms.

Waterfall and gorges

Waterfalls are when the river has a steep fall over a ledge and falls into the plunge pool below. It forms when two types of rock are layered horizontally, with the hard rock on top and the soft below. The soft rock is eroded more quickly than the hard rock, so the river is undercutting the hard rock, leaving an overhang. The overhang falls into the plunge pool as it becomes unsupported and helps with the erosion of the soft rock in the plunge pool, making it deeper. As the process is repeated, the waterfall retreats upstream, and a steep-sided valley, called a gorge, is left.

Fig. 1 - A diagram of a formation of a waterfall

Interlocking spurs

To understand what interlocking spurs are, let's break down the word. Spurs are pieces of land that jut out and cause the river to meander. So, interlocking spurs are when those spurs are projecting from one side after another like the teeth of a zip or when you clasp your hands together and interlock your finger.

Fig. 2 - Interlocking spurs in Ashes Hollows, Shropshire

Erosional and depositional landforms

In the middle course of the river, there are both erosional and depositional landforms, let's take a look into them.

Meanders

Meanders are a series of regular curves in the river channel. In the middle course of the river, there is more lateral erosion which widens the river.

Meanders start off as straight channels. However, there will be obstructions to the flow of water such as boulders and piles of sediment so the water twists and turns creating deeper pathways called pools, and shallow pathways called riffles. As pools are deeper there is greater erosion due to the energy build-up because there is less friction. The shallower areas that are riffles are created by the deposition of coarse sediment. When the pools and riffles form, the flow of the river goes side-to-side in a winding course.

The meander is further shaped by a process called helicoidal flow. It is when the water flows in a corkscrew-like movement with the water hitting the banks like a corkscrew and flowing along the river bed to then deposit the material on the inner bank. The water is faster and has more energy to erode on the outside bend and forms a river cliff. On the inside of the bend, the water is slower and the river deposits some load, creating a gently sloping slip-off slope or river beach. The erosion of the outer banks and deposition of the inner banks will continue to make meanders in the river.

Oxbow lakes

Oxbow lakes can develop from meanders, they form when the river finds a shorter different course. As the erosion of the outside bend and the deposition of the inside of the meander continues, the erosion can narrow the neck within the meander. When there is a flood or a high discharge the river can cut across the neck of the meander. The river then has a straighter, shorter path, and deposition will occur on the banks cutting off the original meander and leaving an oxbow lake.

Fig. 4 - An oxbow lake in Klympen, Denmark

Interlocking spurs

Interlocking spurs are found in the upper course of the river. There is more vertical erosion as the valleys and river channels are narrow, at higher altitudes and the river has a lot of energy.

Let's look closer into how interlocking spurs are created!

Interlocking spur diagram

Interlocking spurs are formed with V-shaped valleys. The river starts off as a small stream in upland valleys with low discharge. So most of the energy that the river has is used to move around the high hillsides and erode vertically through abrasion and hydraulic action. The sides of the channel are unsupported and weak as the river erodes vertically, and is weakened by erosion which creates a steep V-shape in a V-shaped valley. As the land is made of different rocks with different resistance, the river moves around the more resistant rock and erodes the weaker rock leaving interlocking spurs. Take a look at the diagram below.

Interlocking spur characteristics

Interlocking spurs have characteristics such as:

• Steep slopes
• Alternating spurs protruding from opposite sides
• Convex slopes
• Has a narrow valley floor separating the two sides taken up by a river channel
• Many of the slopes are made out of scree

River erosion types

We have touched upon the river erosion landforms and how the river erodes the landscape, with processes of lateral erosion which is the wearing away sideways of the landscape with the river which can be seen in meanders, and the process of vertical erosion which is the landscape being eroded downwards as the water is pulled by gravity which is apparent in interlocking spurs. Scaling down to the processes of erosion that happen in the river and between the river and the river banks and beds, there are four types of river erosion. This is through hydraulic action, abrasion, attrition, and solution.

Hydraulic action

Hydraulic action is when water smashes against the banks of the river and the air gets trapped in the cracks of the river bank. When the pressure of the air is compressed in the cracks, the rock cracks and can break off. This can lead to the river banks collapsing. As we saw in meanders, the velocity of the water is high on the outer bend of the river and hydraulic action removes material on the river banks.

Abrasion

Abrasion happens when the sediment that the river is carrying, like the pebbles and rocks, grinds the river bed and river banks. If there are depressions in the river bed, sediments can get caught and spin around to turn into potholes.

Attrition

Attrition is when the rocks transported by the river knock against each other and break into pieces. The edges of the pieces are smoothed and rounded by this process. Attrition makes the particles smaller and doesn't erode the bank or the bed of the river.

Corrosion

Corrosion happens when the chemicals in water dissolve certain rocks, this could be rocks such as limestone and chalk.