(b) (i) Describe the processes of hydraulic action and corrosion (abrasion) carried out by running waters (rivers). (4 mks)

(c) (ii) Explain how the following river features are formed: - levees - river cliffs (undercut slopes) (8 mks)

The second one comes from the June 2001 paper:

(b) (i) How does a river use its energy? (3 mks)

(ii) Describe two ways by which a river transports its load. (4 mks)

How well did you do in finding the answers? I hope you made some effort to do work. In the first lesson reviewing rivers, I indicated the three activities of the river - erosion, transportation and deposition. They are all interrelated. Deposition and two river features resulting from that activity were already discussed. The questions above refer to the other two activities, that of erosion in the first question and transportation in the second question. Both go on simultaneously. Before discussing the questions, let us review these activities.

EROSION

Streams are able to do their work because they possess energy. There are three things that must be noted in discussing the erosive work of the river. These are its volume, velocity and load. Do you remember what each is referring to? Just in case you have forgotten, here it is in a nutshell:

Volume - The amount of water present in the stream. This varies from source to mouth and also seasonally.

Velocity - The speed of flow of a river.

Load - All the materials, solids or in solution, which are moved by water downstream.

Several interacting processes carry out erosion by the river. They are:

(i) Solution or corrosion
(ii) Hydraulic action or quarrying
(iii) Abrasion or corrosion
(iv) Attrition
(v) Impaction

The first question above asks you to describe the processes of hydraulic action and corrosion. Let me focus on these two, however, take time out to remind yourselves of the other processes. I share with you the information taken from Certified Physical and Human Geography by Goh Cheng Leong.

HYDRAULIC ACTION

This is the mechanical loosening and sweeping away of materials by the river water itself. Some of the water splashes against the river banks and surges into cracks and crevices. This helps to disintegrate the rocks. The water also undermines the softer rocks with which it comes into contact. It picks up the loose fragments from its banks and bed and transports them away.

CORROSION OR ABRASION

This is the mechanical grinding of the river's traction load against the banks and bed of the river. The rock fragments are hurled against the sides of the river and also rolled along the river bottom. Corrosion takes place in two distinct ways:

(a) Lateral corrosion - The sideways erosion which widens the V-shaped valley.

(b) Vertical corrosion - The downward action which deepens the river channel.

The second part of sub-section (c) of the same question is asking for an explanation on the formation of river cliffs (undercut slopes). These are formed on the outer curve of a river meander, where lateral erosion is strongest. When the flow of water enters the bend in the river, the concave bank, erosion occurs. This erosion of the outer bank of the river over time, results in the formation of a steep river-cliff. Refer to the diagrams below. (Source: Certificate Physical and Human Geography: Goh Cheng Leong (p52).

The second question will be discussed in the next lesson.