Ch 2.3 How do fluvial processes shape the land? Flashcards
What are the landforms in the upper course?
1, Interlocking spurs and V shaped valley.
2. Rapids, waterfalls and gorges.
3. Plunge pools
4. Pot-holes.
How is the interlocking spurs and V-shaped valley formed?
Vertical erosion is active at the upper course. Over time, a narrow steeped sided V-shaped valley is formed. On the other hand, the river water is unable to remove the obstacles. River flows side to side around the obstacles thus forming interlocking spurs.
How are the rapids, waterfalls and gorges formed?
When a land of more resistant and less resistant rock lies vertically, horizontally or obliquely across the river, erosion will be concentrated at the less resistant rock. Small steps develop due to fluvial erosion.
When there’s continuous erosion on less resistant rock, great differences are created in water level between hard and soft rocks. The waterfall is formed.
The strong hydraulic action enlarges the notch at the back of the waterfall. Without support, overhanging rock collapses. When the process repeats over time, waterfalls retreat and form a well-developed gorge. It is a valley with narrow, deep and steep-sided channels.
How are the plunge pools formed?
Once the waterfall forms, the force of running water can open/undercut the cracks at the base and back of the waterfall. It forms a plunge pool by hydraulic action.
How are pot-holes formed?
When a river flows over an uneven surface, it causes turbulence in the water. The turbulence causes rocks or pebbles to drill hollows on the river bed. It is called potholes.
What are the landforms in the middle course?
Meanders & River bluffs
How are the meander and river bluffs are formed?
As the meanders migrate downstream, they remove the interlocking spurs on their way. River cliffs are pushed back and join into lines called bluffs.
River flows faster at the outer bank or concave bank, thus erosion takes place. Lateral erosion will erode the river bank. At the inner bank, river flows slowly. Deposition occurs. Sediment may deposit on the river bank. And the result is that water moves from side to side in a wave like pattern. It is a winding course. It is called Meanders.
What are the landforms in the lower course?
- Floodplains and levees.
- Ox-bow lakes.
- Braided streams
- Deltas and distributaries.
How are the flood plains and levees formed?
In the lower course, the river bed is raised by deposition. The load comes from erosion at the upper course. During rainy seasons, the river floods and overflow its banks. Flood water carries sediments to the surrounding areas. Coarser sediments are deposited near the river banks, called levees.
Finer sediments are spread further. When dry seasons come, evaporation takes place. Water is evaporated or drained by farmers. After repeated flooding, floodplains are built up.
How are ox-bow lakes formed?
Active lateral erosion pushes the outer banks of the river towards each other. River water cuts across the narrow neck between two outer banks. The two outer banks meet, a new channel is formed. The abandoned channel will be sealed by deposition and form an oxbow lake.
How are braided streams formed?
Due to decrease in discharge, gentle slope gradient and the obstruction of large amounts of sediments, a river is divided into small channels. These channels may split and join the mainstream finally.
What are the characteristics of braided streams?
Wide and shallow.
Patterns varies with time as the sediment ridges get eroded away and deposited to form new ridges.
How are deltas and distributaries form.ed?
Delta is the accumulation of sediments at the river mouth. It is formed by the deposition of sediments.
Active downward erosion in the upper course that produces a large amount of sediments. When the river enters the shallow open sea. Sea water is very calm, the tide is weak and there is no strong ocean current. The sediment deposited and accumulated. A low-lying area of sediments will gradually develop. It may rise above the sea level and form a delta like Zhujiang Delta.
With more sediments, the mainstream is blocked. Repeating blocking and breaking through extends the deposit seaward. A fan-shaped and gently sloping platform forms distributaries.
