Summary
Highlights
Entrenched or incised meanders occur when a meandering river experiences rejuvenation. The increased vertical erosion causes the meanders to cut deeper into the valley, creating distinctive, deep, winding channels.
A valley-in-valley feature is formed when a rejuvenated river cuts a new, smaller valley within a pre-existing, larger valley. This process involves initial cutting, followed by some lateral erosion, and then further vertical erosion during subsequent rejuvenation events.
Features indicating river rejuvenation on topographic maps or aerial photos include waterfalls, oxbow lakes, incised meanders, and river terraces. The presence of any of these signifies that rejuvenation has occurred in the area.
The video analyzes a past paper question, identifying vertical erosion as the primary type of erosion associated with rejuvenation. It also highlights how upliftment and incised meanders are evidence of rejuvenation, and discusses reasons why rejuvenated land might not be suitable for human activity, such as steep slopes and narrow floodplains.
Rejuvenation significantly changes downstream fluvial features. Knickpoints and waterfalls develop, vertical erosion leads to terraces and valleys-in-valley, and meanders become deeply incised. Floodplains may narrow, and a higher velocity can remove braided streams, break through levees, and create more oxbow lakes, all demonstrating the change in landscape due to rejuvenation.
The video introduces the concept of river rejuvenation, which involves an increase in a river's speed and erosive power, leading to increased downward or vertical erosion. This often occurs due to a change in gradient, making the river more energetic. The key characteristic is the deepening of the river channel.
River rejuvenation can be caused by: a drop in sea level, which steepens the river's gradient; land uplift due to tectonic forces or isostasy, also creating a steeper gradient; an increased volume of water, such as from higher rainfall; or river capture, where a river captures the water of another, increasing its volume and speed.
Knickpoints are features formed by rejuvenation, marking a sharp change in the river's channel slope or gradient. This change leads to an increase in downward erosion and can result in the formation of waterfalls or rapids.
River terraces are step-like formations on either side of a river valley. They form when a rejuvenated river cuts deeply into its valley floor. Subsequent rejuvenation events lead to further vertical erosion, creating multiple steps at different levels.