Have you ever stood in front of a waterfall wondering how the drop was formed or pondered why the river took the path that meanders through interlocking valleys whilst hiking? Both of these curiosities are linked to the way that the river erodes the landscape and shapes it. Consequently, river erosion landforms are created. But what is the difference between erosion and depositional landforms? Are there different types of erosion? Let's look into the different landforms created through erosion and how they are formed!
River erosion and depositional landforms
River landforms are created from the movements of the river on the surrounding landscape. There are two types of river landforms, erosional and depositional landforms; sometimes, there is a mixture of both to form one landform.
River erosional landforms are physical features of the land that are formed from the river wearing away the land.
River depositional landforms are the land's physical features formed by the river-dropping material it carries.
Take a look at our River Depositional Landforms to learn more about deposition!
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
We will look into more detail about interlocking spurs at the end of this article.
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.
Lateral erosion is when the river erodes the landscape sideways.
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.
You can learn more about types of river erosional processes later in the article.
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.
Vertical erosion is when erosion happens in a downwards direction. For example, a river that is falling from a higher place eroding vertically from the pull of gravity. Vertical erosion can also form V-shaped valleys.
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
Scree is pieces of rock that have broken off the side of the mountains from freeze-thaw weathering.
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.
Erosional processes can be confused with transportation processes so have a look at the article 'River Processes'.
River Erosion Landforms - Key takeaways
- River erosional landforms are when the river forms physical features from wearing the land away. River depositional landforms are when the river drops the material it was carrying to create physical land features.
- In the upper course of the river are erosional landforms such as waterfalls, gorges and interlocking spurs with erosional and depositional landforms in the middle course of the river such as meanders and ox-bow lakes and depositional landforms in the lower course of the landforms such as floodplains, levees and estuaries.
- Interlocking spurs are a type of river erosional landform that is formed with V-shaped valleys through vertical erosion.
- There are four types of river erosion, hydraulic action, abrasion, attrition, and corrosion.
References
- Fig. 1 - A diagram of a formation of a waterfall (https://commons.wikimedia.org/wiki/File:Waterfall_formation23.png) By Jerry Crimson Mann (https://commons.wikimedia.org/w/index.php?title=User:Mcy_jerry&action=edit&redlink=1) Licensed by CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/)
- Fig. 4 - An oxbow lake in Klympen, Denmark (https://commons.wikimedia.org/wiki/File:Finna2.JPG) By Bjørn Ove Finseth (https://commons.wikimedia.org/wiki/User:Boffe) Licensed by CC BY 2.5 (https://creativecommons.org/licenses/by/2.5/)
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