River deposition

THE THREE river processes identified are erosion, transportation and deposition. In lessons prior to this, I shared information on the processes of river erosion and transportation. Today, I will go on to the third river process, that of river deposition. Deposition is defined as the progressive laying down of material by the river and is complementary to erosion. Material removed from one place is transported and deposited at another place.

Rivers will transport their load as long as they have the energy to do so. If the river loses its energy, there will be deposition of the load on the riverbed. When I shared with you the information on river transportation, I mentioned two terms, namely, competence – a measure of a river’s ability to carry load; and capacity – the ability of the river to carry a certain amount of materials; in other words, the total load that the river can carry. In essence, both are decreased, resulting in the river dropping its load.

Some of the causes of such decreases are: (a) reduction in stream gradient

(b) decrease in volume

(c) loss of velocity

(d) overloading

(e) freezing, and

(f) emptying into quiet or slower-moving bodies of water.

The order in which the materials are deposited is – boulders, pebbles, gravels, sands, silts, and muds, although the change from coarse to fine material is normally imperceptible. The heavier materials are deposited usually at the upper course of the river. Finer particles travel greater distances downstream before deposition occurs. Deposition is not confined to the lower courses of rivers. Much deposition does occur there, but some may occur at almost any point along the river’s course, such as on the inside bend of the river meander, on a floodplain, or at the river mouth.

As you revise this topic of river processes, include the different physical features that are formed as a result of river erosion and river deposition. You must be able to describe their formations, draw and label diagrams of them, and give examples where they can be found.

Let us now turn our attention to wave processes, to complete the specific objective 11

which states that students should be able to ‘describe river and wave processes’. One of the three movements associated with seawater is waves. Waves are produced by the friction of wind on open water. Essentially an up-and-down movement of the water, wave motion also moves the surface water in the direction that the wind is blowing. Breakers are formed when the wave comes into shallow water near the shore. The lower part of the wave is retarded by the ocean bottom, while the top, having greater momentum, is hurled forward, causing the wave to break.

Waves are the most powerful agents of marine erosion. As they attack the shore, they erode it by a combination of several processes, namely, corrasion, hydraulic action, solution and attrition, processes that are similar to those which occur in river erosion. Each of these will be discussed below.

Corrasion is also called abrasion. Corrasion involves the rock fragments being carried by the waves towards the coast. The erosive work of waves, like that of running water, will clearly be greater when they are provided with cutting tools. Waves, armed with rock fragments of all sizes and shape, are able to undertake their erosive work much more quickly and effectively. These rock fragments can be as huge as boulders. The energy of the waves determines the size that they can carry.

Hydraulic action – The force of waves, as they batter against cliffs and seawalls, exert considerable pressure upon the coast. In their forward surge, waves splashing against the coast may enter joints and crevices in the rocks. The air imprisoned inside is immediately compressed. When the waves retreat, the compressed air expands with explosive violence. Such action, repeated again and again, soon enlarges the cracks and rock fragments are

prised apart.

Information on the other processes will be shared in the next lesson.

Here is a study tip: ‘Study for tests together, ask questions and teach each other.’

REFERENCES

CSEC New Integrated Geography – Sheldon G. Bleasdell

Certificate Physical and Human Geography – Goh Cheng Leong

Geology Made Simple – William H. Matthews Geography for CSEC – Jeanette Ottley, et al Morphology and Landscape – Harry Robinson