Overview
This lecture focuses on the concept of river rejuvenation, exploring its causes, key features, and how to identify and explain its effects on river valleys.
Exam Guidelines
- Exam covers the concept of river rejuvenation, reasons for it, and resulting features.
- Practice with past exam questions is recommended.
River Rejuvenation: Concept and Causes
- River rejuvenation is when a river gains energy, increasing vertical (downward) erosion.
- It occurs when a river’s speed and erosive power increase, usually due to a steeper gradient.
- Main causes include sea level lowering, land uplift (tectonic forces), increased water volume (e.g., more rainfall), and river capture.
Key Features Formed by Rejuvenation
- Knickpoint: Sharp change in river gradient, often leading to waterfalls or rapids.
- River terraces: Step-like features formed as the river cuts successively deeper into its valley, creating paired terraces on both sides.
- Incised/Entrenched meanders: Deeply cut river bends formed by vertical erosion following rejuvenation.
- Valley-in-a-valley: A smaller, deeper valley formed within an older, wider valley due to repeated rejuvenation and erosion.
Identifying Rejuvenation in Maps and Photos
- Look for waterfalls, oxbow lakes, incised meanders, and river terraces on maps or photographs as signs of rejuvenation.
Effects and Suitability for Human Use
- Rejuvenated land has steeper slopes and deep valleys, making farming, building, and infrastructure difficult and expensive.
- Narrow floodplains mean less fertile land and limited access to water.
Sample Exam Questions & Answer Strategies
- Vertical erosion is the main process during rejuvenation.
- Look for uplifted land and incised meanders as evidence of rejuvenation.
- Causes of uplift include tectonic movement, isostatic adjustment, or folding.
- Explain how features like knickpoints, terraces, and valley-in-a-valley develop downstream after rejuvenation.
Key Terms & Definitions
- River Rejuvenation — When a river gains renewed erosive power, causing increased downward erosion.
- Knickpoint — A point of sharp change in channel slope.
- River Terraces — Step-like landforms on valley sides formed by repeated river downcutting.
- Incised/Entrenched Meander — A river bend that is deeply cut into its valley after rejuvenation.
- Valley-in-a-Valley — A smaller, deeper valley carved within an older, wider one due to successive rejuvenation.
- Vertical Erosion — Downward cutting by a river into its bed.
Action Items / Next Steps
- Practice identifying features of rejuvenation on topographic maps and photos.
- Answer past exam questions on river rejuvenation and its effects.
- Review the definitions and key features for upcoming assessments.