- Explain why V-shaped valleys often contain rapids and/or waterfalls.
- Describe or sketch how an erosional floodplain develops.
- List two situations that would trigger the formation of incised meanders.
The Correct Answer and Explanation is :
1. Why V-shaped valleys often contain rapids and/or waterfalls
V-shaped valleys are typically formed by the erosion of river systems over time. They often contain rapids and waterfalls due to the steep, narrow channel formed by the erosional processes within the valley. In these valleys, the river or stream has high energy, which is a result of the steep gradient or slope of the valley floor. The steep gradient increases the velocity of the water flow, causing it to have a higher erosive power.
- Rapids are formed in these environments when the river flows over resistant bedrock or uneven surfaces, creating turbulent water flow. The water may encounter boulders or irregular rock formations that disrupt its smooth flow, causing the water to move chaotically, which results in rapids.
- Waterfalls can form where there is a sudden drop in elevation or a contrast between layers of hard and soft rock. The softer rock erodes faster than the harder rock, leading to a vertical drop in the river’s course, creating a waterfall. In a V-shaped valley, the steep sides make waterfalls more likely, as the river is confined in a narrow channel and has less space to spread out horizontally.
Thus, the combination of steep gradients, variable rock types, and the high energy of the flowing water makes rapids and waterfalls common in V-shaped valleys.
2. How an erosional floodplain develops
An erosional floodplain typically develops through the lateral (sideways) erosion of a river channel over long periods of time. In regions where a river is meandering, the floodplain forms by the erosion of the river’s banks during flood events and the deposition of sediment in areas of lower velocity. The following steps outline the development of an erosional floodplain:
- Initial River Formation: The river begins to erode the landscape as it flows, cutting into the bedrock and forming a channel. In its early stages, the river may have a steep gradient, leading to rapid vertical erosion.
- Lateral Erosion: As the river matures, the gradient becomes gentler, and lateral erosion (erosion of the riverbanks) becomes more prominent. The river’s flow starts to meander across the valley, eroding the outer bends of the meanders and depositing sediment on the inner bends.
- Flooding: During times of high water (usually seasonal flooding), the river overflows its banks. The floodwaters carry away sediment from the channel and deposit it on the surrounding land, creating a wider floodplain.
- Sediment Deposition: Over time, as the river shifts and deposits sediment, the floodplain grows. The process is repeated, with more sediment being added to the floodplain every time the river floods.
As the floodplain develops, it becomes a broad, flat area that is regularly inundated by floodwaters and continues to evolve through erosion and deposition.
3. Two situations that would trigger the formation of incised meanders
Incised meanders are formed when a river that was once flowing across a flat or gently sloping landscape begins to carve deep, steep-sided valleys into the underlying bedrock. This can occur under two main situations:
- Fall in base level: A river erodes its bed in response to a fall in its base level, which could be due to a lowering of sea level, tectonic uplift, or the formation of a dam or other obstruction. The river’s flow continues in its existing path, but because the base level has dropped, the river has more energy to erode vertically. This increased erosion deepens the river channel, causing the formation of incised meanders as the river cuts downward into its floodplain or valley floor.
- Climate change or tectonic uplift: A shift in climate, such as a long-term decrease in rainfall or an increase in the rate of sediment transport, can lead to river incision. Similarly, tectonic uplift can raise the surrounding landscape, causing a river to erode downward to maintain its slope and energy. These changes trigger incised meanders, as the river cuts deeper into its original path in response to the new conditions, leading to the formation of steep-sided meander loops.
In both situations, the river gains enough energy to start cutting deeper into its own channel, creating the characteristic steep walls of incised meanders. These meanders are often found in regions that have experienced changes in base level, climate, or tectonic forces over time.