River rejuvenation is a natural process that occurs when a river resumes its downward cutting and erosion into the landscape after being temporarily slowed or stopped by various factors.
The process of river rejuvenation is associated with two main types of erosion:
Vertical Erosion: During river rejuvenation, the river resumes its downward cutting into the landscape, leading to the deepening of the riverbed. This is known as vertical erosion. As the river cuts deeper, it creates a V-shaped valley, and the river channel becomes more pronounced.
Lateral Erosion: Along with vertical erosion, river rejuvenation can also lead to lateral erosion. Lateral erosion occurs when the river widens its channel by eroding the banks and adjacent land. This can result in the formation of river meanders and the shifting of the river’s course over time.
Both vertical and lateral erosion contribute to the overall process of river rejuvenation, which helps the river to adjust its course and resume its journey towards the sea or lower elevations. River rejuvenation can be triggered by various factors, such as changes in tectonic activity, uplift of the land, changes in climate, or adjustments to the base level of the river (e.g., due to changes in sea level). As the river resumes its erosion and downcutting, it can lead to the formation of new landforms and alter the landscape over geologic time scales.
Let’s consider an example of river rejuvenation:
Imagine a river flowing through a mountainous region. Over time, the river has cut down through the landscape, forming a V-shaped valley. However, due to tectonic uplift in the region, the land is gradually rising, and the river’s downward cutting is slowed or stopped. As a result, the river starts to meander across a relatively flat floodplain.
After a period of uplift, the tectonic forces causing the land to rise diminish, and the region experiences a period of tectonic stability. During this stable phase, the river is no longer being uplifted, and its base level (the level of the body of water into which it flows, typically the sea) remains unchanged.
With the uplift ceased, the river begins to adjust to the new stable conditions. The river’s flow becomes more efficient, and it starts to regain its downward erosional power. As a result, the river resumes its vertical erosion, cutting deeper into the landscape.
As the river deepens its channel, it forms a new V-shaped valley, cutting through the previously formed meanders. The meandering pattern of the river is disrupted, and the river starts to erode and widen its banks laterally.
Over time, the river rejuvenation leads to the formation of a new river valley with steep sides and a pronounced river channel. The previously meandering floodplain may be abandoned as the river cuts through it, leaving behind remnants of old meanders as oxbow lakes or wetlands.
The rejuvenated river’s course now follows a more direct and steeper path, allowing it to carry sediment more efficiently and erode the landscape effectively. This process continues over geological time scales, shaping the landscape and contributing to the ongoing evolution of the river system.
In this example, the river rejuvenation is driven by changes in tectonic activity, and the result is the formation of a new valley with vertical and lateral erosion playing essential roles in reshaping the river’s course and the surrounding landscape.