Soil erosion is a process by which the earth’s crust wears away due to external dynamic agents such as water, wind, gravity and ice. It involves the breaking down of rock particles along the river bed and banks, and these particles are carried downstream by the flow of the river.
Four main forms of river erosion:
- Hydraulic action: air trapped in the cracks and crevices of the riverbank due to the force of the river weakens the banks
- Abrasion: particles carried by the river, wear down the bed and banks
- Attrition: rocks being carried by the river, rub against each other making them smaller and smoother
- Solution: soluble particles are dissolved into the river
Erosion can also be caused by the falling and slowing of water:
- Raindrop erosion: Erosion resulting from the direct impact of falling drops on soil particles
- Sheet erosion: Removal of a layer of exposed surface soil due to the splashing and run-off of raindrops
- Rill and Gully erosion: Rivulets that cut several inches into the soil surface
- Stream and Channel erosion: erosion in the stream bank and bed due to an increase in the volume and velocity of runoff
In this research assignment we were allocated a section of the Don riverbed to comment on the stabilization of the river banks. We also analysed the trail along the riverbed and commented on the conditions. The riverbed we were assigned was the ‘Beginning of segment 4’ and the walking trails. The photographs were taken on the 23rd of November 2016, it was -2°C around 2:30pm. There had been a snowfall the previous night and thus the ground was muddy.
Figure 1. Google map of Don River and the segments each group had to take photographs of pin pointed. The purple line is the trail that was photographed and addressed in terms of conditions. The 3 stars represent the three pictures of the Trails addressed below.
Figure 2. A picture of the trail marked with a blue star marked on the Google map (Figure 1).
Figure 3. A picture of the trail marked with a yellow star marked on the Google map.
Figure 4. A picture of the trail marked with a orange star marked on the Google map.
As observed in the pictures, the trail was mostly muddy due to the snow and rain falls and there are noticeable patches of snow. There were also many leaves that had fallen and covered most of the path. In the first picture there are many trees and plants on either side of the path and the path is also wider. The trees on the riverbank are mostly leaning in towards the river, showing signs of bank erosion. In figure 3 you notice the stones that align along the path on the side of the riverbed. The trees in this picture are larger and elevation between the trail and the riverbed has decreased, there is a larger surface area of river bank between the trail and the river. In the last picture the train is right up against the river, the division between the river and the trail are the large rocks that align the river bank. The bank is steep and there is no vegetation on the bank.
The riverbank had a larger surface area and less erosion when there were means of river bank stabilization. For instances an increased number of large trees and more vegetation increased river bank stability, likewise trees were planted in a watershed in Nashwaak to increase river bank stability. The large rocks were put in place to further stabilize he bank, they prevent the bank from further erosion.
Riverbank segment 4
Figure 5. Photograph of the meander, from segment 4 of the riverbed.
Figure 6. Photograph of a stretch of the river after the meander. Both sides of the riverbed are seen in this picture.
Figure 7. Photograph following the meander, the vegetation on the opposite end of the riverbed is also shown.
Figure 8. A diagram obtained from a website called ‘Geography AS Notes’ and a post on River Landforms by Alex Jackson. Shows the formation of Oxbow lakes as a result of the presence of meanders.
Segment 4 of the riverbed included a meander. As observed in figure 5, there is deposition on the inner part of the meander and a slip-off slope is visible. There is erosion on the river cliff of the meander and the presence of vegetation and many medium sized rocks which act as a form of bank stabilization, reducing erosion. Over a long period of time, this erosion and deposition may form a meander as observed in figure 8. Figure 6 and 7 are pictures following the meander and as observed there is continuous erosion and deposition on the river bank. The side that is eroding is of higher elevation than the side that is undergoing deposition. The vegetation on the bank of the river, act as a temporary form of stabilization of the river bed.