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WEEK 2:
TOPIC: ACTION OF RUNNING WATER
CONTENT:
(1) Erosional features of a river
(2) Depositional Features of rivers
SUB-TOPIC: (1) EROSIONAL FEATURES OF A RIVER
The upper course of a river begins at the sources of the river near the watershed. The flow is very swift as it descends the steeps and the action of the river is predominantly vertical erosion. The valley developed is thus deep, narrow and distinctively V-shaped. In some cases where the bed rock is hard and resistant, vertical erosion carves very deep and narrow valleys called GORGES.
GORGES OR RAVINES: The gorges can exist in almost any part of the river course, provided the conditions are suitable. Gorges are narrow, steep sided valleys. They are predominantly found in upper course of a river. Where vertical erosion is greater than lateral erosion. Examples of these features are Shiroro Gorge on River Kaduna in Nigeria, the Volta Gorge in Ghana and the Rhine Gorge in Germany. Gorges provide good sites for building dams across rivers to generate electricity.
V-SHAPED VALLEY: This is formed as a result of vertical erosion of the river. The valley is deepened by vertical corrasion and widened by weathering and mass wasting which causes it to become V-shaped.
RIVER CAPTURE: This is also known as river piracy or river beheading. It is a process by which a river acquires the headstreams of another river and thus enlarges its own drainage area at the expense of the other. The weak rivers are deprived of their tributaries; they lose part of their basins and can be totally disconnected to original streams or rivers. Example is Ogunpa streams in Ibadan city of Nigeria. Another is river Tilden Fulani which has cut through the weak link between the Naraguta hills and Shere hills in Jos Plateau State of Nigeria. The rate at which a river erodes its beds depends largely upon its own speed and the nature of the underlying rocks. The development of a sharp almost right-angled bend along the course of the river is called elbow of capture which is common evidence of river capture. Another evidence of river capture is the occurrence of a dry gap in the high ground opposite the elbow of capture. The presence of a misfit in the lower section of a dry gap also strengthens the proof that a river capture has taken place. Misfit is a stream that is too small for its valley.
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