Students of Pentab High School, located at 16 North Street, Kingston. - Inan Allen Photo

(a) Study the diagram at right, which shows the features of a river, and answer the following questions:

(i) Identify the features marked I, II and III. (3 marks)

(ii) Name the section of the river where these features are likely to occur. (1 mark)

(b) (i) Explain how any TWO factors can cause river flooding. (4 marks)

(ii) Describe TWO measures used to reduce river flooding. (6 marks)

(c) Explain how a river may form a lake on its flood plain. (4 marks)

THE RESPONSE IS AS FOLLOWS:

Part (a) - (i) The features identified on the diagram are: I - levee; II - tributary; III - flood plain.

(ii) The section of the river where these features are likely to occur is in the lower valley or the course/plain stage.

Part (b) (i) - There are several factors that can cause river flooding. Although the question is asking you to explain only TWO, I will share with you several factors so that you can be aware of them. Naturally, in the examination, you would write about two only.

In referring to the matter of river flooding, let me state here that this results whenever the channel capacity is exceeded by the run-off. If there is more water present in the river channel than what it is able to carry, then the water will go beyond the banks of the river, and flooding will occur. This is a temporary rise in the level of water in the river and there are several contributing factors for this situation. These are tabulated below:

1. Unusually heavy rainfall that leads to excessive run-off of rainwater.

2. Excessive run-off of water as a result of snowmelt.

3. A long period of rainfall, which causes the earth to become satura ted and therefore increases overland flow of water.

4. There may be barriers in the river, such as dams, debris, garbage and material from landslides. These affect the flow of the river.

5. A sudden release of water due to a broken dam.

6. The removal of vegetation on river valley slopes as well as poor farming practices will lead to soil erosion. This can cause an excessive amount of soil to be washed down to the river channel so silting the riverbed.

7. Other activities of man such as sand mining and quarrying, especially if these are not done properly, can lead to flooding.

Part (b) (ii) of the questions requires that you describe two measures that can be used to reduce river flooding. Again, I will share with you, more than the two:

1. The planting of a cover of vegetation to reduce the concentration of surface run-off during a downpour.

2. The building or levees and dikes downstream to contain the surge of floodwater.

3. The river's course to the sea may also be shortened by the construction of channels which bypass bends and meanders in the river. This speeds up the discharge of water.

4. The dredging of the channel. This helps to deepen the channel to avoid excessive sedimentation.

5. The terracing of slopes. Terraces will hold back some of the soil from the slope so lessening the problem of silting.

6. Removing the garbage and other materials from the channel to allow the free flow of water.

Certainly, I have not exhausted the list of measures that can be taken to alleviate the problem of flooding. See how many others you can find through your reading.

Part (c) of the question is asking you to explain how a river may form a lake on its flood plain. This is referring to the formation of an oxbow lake. In the lower course of a river, a meander becomes very much more pronounced. The outside bend or concave bank is so rapidly eroded that the river becomes almost a complete circle. There will come a time when the river cuts through the narrow neck of the loop, abandoning an oxbow lake. The river then flows straight.

Remember, your diagrams are useful for this explanation.