Unit Topic: The Work of Rivers
Sub-Topic: Stages of a River
Duration: 70 minutes
At the end of the lesson students will understand the stages of a river.
At the end of the lesson students should be able to:
- State the three different stages of a river after watching a video.
- Describe each stage of the river after reading from handout and textbooks.
- List the processes that that are involved in each stages of the river after discussion.
- Describe the load found in each stage after viewing pictures.
- Draw diagrams to show the three stages of the river after viewing pictures of them on the screen.
- Appreciate the distinct characteristics of a river.
- The three stages of a river are: the upper course/ Youthful stage, the middle/ mature stage and the lower/ old stage.
- The upper course or youthful stage
- The gradient in this section is very steep
- The river flows very fast at this section
- Vertical down cutting takes place in this stage/ vertical erosion
- Produces interlocking spurs
- Large boulders are found in this course
- The middle course or mature stage
- Lateral erosion takes place in this course
- Gradient is gentler in this stage
- Some deposition takes place in this stage
- The velocity of the water flow decrease
- Meanders start to develop
The lower course or old stage
- Flow is even more slower in this stage
- The gradient in this stage is much gentler
- Valley floor is very wide
- Meanders are well developed
- Great deposition takes place here
Features found in each stage
- Narrow V-shaped valley: This is due to the river cutting down into its bed, with weathering and mass-movement operating slowly along the valley sides.
- Interlocking spurs: These are ridges of high land descending from the mountain to the valley. The river will flow around these outcrops of resistant rock on its way downstream.
- Potholes: These are circular depressions found on the river bed caused by pebbles being carried by the river becoming trapped on the uneven surface and so will swirl around to form potholes.
- Gorge: This is a narrow, steep sided river valley where the rate of vertical erosion is faster than the rate of weathering and mass-movement on the valley sides.
- Waterfall & Rapids: A waterfall results from differences in the strength of rocks.
- Open V-shaped valley: Here lateral erosion becomes more pronounced, with weathering and mass-movement processes operating faster along the sides.
- River cliffs: The river in this stage begins to meander. At a meander bend there is erosion on the outer bank because the river is faster flowing forming a river cliff or bluff.
- Flood plain: During a flood when the river overflows its banks, it spreads out and deposits fine material over the flood plain which then gradually builds up after successive flooding periods.
- Leveés & deferred junctions:
Leveés are raised river banks built up by deposition of coarser material during a flood. This material builds up above the level of the surrounding flood plain.
- Oxbow lake: This crescent-shaped lake forms where there is a marked meander with a narrow neck. During a flood the neck may be cut through, as the river tries to follow a more direct route.
- Braiding: During dry periods when the river is not able to carry its full load, deposition may occur on the bed of the river.
- Delta: This is an accumulation of successive layers of alluvium deposited at the mouth of the river.
Students will be asked to watch a short video showing a tutorial on the different stages of the river. Students will be asked to say what they think the lesson will be about based on what they have seen in the video. The answer that is expected is “river stages”. From this activity students will know what the topic of the lesson is.
Step 1: Students will be asked to observe what they see in the video and state the different stages of a river. The stages will be listed on the whiteboard as students make mention of them, students will then be allowed to copy them into their textbooks.
Step 2: Teacher will do explanation of each stages of the river with the use of power point presentation and pictures. Students will be asked questions during this explanation and they will be asked to make a note of the important information.
Step 3: Students will then be placed into three different groups and will be given a handout to describe the features that are found in the different stages of the river. Each group will do a presentation on the stages they identified.
Each student will be asked to identify each stages of the river and describe one feature found in each.