Rejuvenation in the Cycle of Erosion
Imagine you are watching a cricket match. The batsman, initially full of energy, starts playing aggressively, hitting boundaries with ease. As the innings progresses, fatigue sets in, and the once-powerful strokes become slow nudges. Just when you think he is about to retire, he drinks an energy booster and suddenly regains his power, smashing the ball like a young player once again.
This is exactly what happens in rejuvenation of rivers—a river that has grown old and sluggish suddenly regains its youthful vigor due to certain geological factors. It begins eroding aggressively, as if nature has given it a second innings!
Let’s dive deeper into this fascinating concept.
Understanding Rejuvenation: The River’s Comeback
A river undergoes a natural cycle of erosion, much like a human life cycle—youth, maturity, and old age:
- Youth Stage: The river is young, energetic, and flows rapidly over steep slopes, cutting deep valleys.
- Mature Stage: The river slows down, widens its valley, and starts depositing sediments.
- Old Age (Senile Stage): The river becomes sluggish, forming broad floodplains, meanders, and delta deposits.
But what if something re-energizes this slow-moving old river? That’s where rejuvenation comes in.
What Triggers Rejuvenation?
Rejuvenation occurs when the erosive power of the river increases due to external factors. Imagine pushing a man down a steep slope—he will suddenly start running! Similarly, when the river gets a steeper gradient due to changes in land or sea level, it starts eroding aggressively again.
Rejuvenation interrupts the river’s natural aging process and pushes it back to its youth stage.
Types of Rejuvenation: The River’s Power Boosters
1. Dynamic Rejuvenation – The Land Rises
Think of a running escalator. If you are walking on it, your speed remains normal. But if someone suddenly tilts the escalator upwards, you have to work harder to climb. Similarly, if land gets uplifted, the river suddenly finds itself on a higher slope, and its speed increases.
✅ Causes:
- Tectonic uplift (like the Himalayan uplift raising river gradients)
- Tilting of land
- Lowering of the river’s outlet (where it meets the sea)
✅ Example: The Chotanagpur Highlands in Jharkhand, where rivers got rejuvenated due to tectonic upliftment.
2. Eustatic Rejuvenation – The Sea Level Drops
Imagine you are walking towards a swimming pool with water at chest level. Suddenly, the water level drops to your knees—you feel taller and start walking faster. Similarly, when sea levels drop, the river suddenly finds itself at a higher elevation and starts flowing more vigorously.
✅ Causes:
- Glaciation: Water gets trapped in ice sheets, lowering sea levels.
- Tectonic movements: Land rising or sea basins sinking.
✅ Example:
- Rivers in Europe and North America were rejuvenated during the Ice Age due to lower sea levels.
3. Static Rejuvenation – The River Gets More Water
Suppose a slow-moving river suddenly receives heavy rain or an extra supply of water from another river. This increased volume makes it erode more actively.
✅ Causes:
- Increase in rainfall (more water = more erosive power).
- River capture (one river stealing another river’s flow).
✅ Example:
- The Kaveri River in South India experienced rejuvenation due to additional water supply from tributaries.
Effects of Rejuvenation: Signs Left by the River
1. Valley in Valley (Two-Storey Valleys)
Imagine an old, broad river valley. Now, if the river is rejuvenated, it starts cutting a new, deeper valley inside the old one—just like building a new road inside an old one.
✅ Example: The Damodar Valley near Rajrappa (Jharkhand), where an old valley was cut deeper due to rejuvenation.
2. Uplifted Peneplains
A peneplain is a nearly flat land formed by erosion over millions of years. Now, imagine if this land gets lifted up due to tectonic movements. It is still flat, but at a higher level—like an old table raised on a new platform.
✅ Example: The Patlands of Ranchi Plateau, uplifted due to Himalayan tectonics.
3. Incised Meanders
Meanders are large loops in a river’s path. When a river is rejuvenated, it starts cutting vertically downward, making the meanders deep and steep. It’s like a snake getting stuck in a trench—it can’t move sideways anymore!
✅ Examples:
- Bhedaghat Gorge of the Narmada River (M.P.) – Famous for marble rocks.
- Karo River (Jharkhand) – Deep meandering valley near Pheruaghaugh Falls.
4. Nick Points
Ever noticed how a staircase has sudden steps? When a rejuvenated river suddenly drops in elevation, it creates a sharp break in its slope. This is called a nick point—a sign that the river was rejuvenated at that point. Therefore, nick’ represents breaks in slope in the longitudinal profile of a river caused by rejuvenation. This is why nick point is also called as head of rejuvenation.
✅ Example: Waterfalls like the Angel Falls or the Jog Falls often form at nick points.
Conclusion: Why Does Rejuvenation Matter?
Rejuvenation is not just a geological curiosity—it shapes the landscapes we live in. It creates valleys, waterfalls, gorges, and terraces that impact agriculture, hydropower, and settlements. Many of India’s river-based civilizations exist because of landforms shaped by rejuvenation.
Sample Question
Q. Explain the concept of topographic discordance in the context of topographic expressions of rejuvenation. How does it indicate multiple cycles of erosion in a landscape? Illustrate with suitable examples. (150 words)
A: Topographic discordance refers to the disruption in the uniformity of a landscape due to rejuvenation, resulting in an older topography above and a younger, deeply incised form below. It occurs when a river, after reaching a mature or senile stage, experiences renewed erosion due to a fall in sea level or land uplift. This leads to the formation of a two-storeyed valley, where a deep and narrow valley is cut within a broad and shallow one.
This phenomenon is a key topographic expression of rejuvenation, indicating multiple erosion cycles (polycyclic reliefs). The Damodar Valley at Rajrappa (Jharkhand) is a classic example, showing distinct layers of valley formation. Other related features include uplifted peneplains, incised meanders (e.g., Bhedaghat gorge of Narmada), and knick points.
Topographic discordance highlights the dynamic interaction between fluvial processes and tectonic activities, offering insights into long-term landscape evolution.
