Jeanell Anderson of St. Andrew High School on her way to winning the Girls Class 3 discus at the Big Shot II Invitational meet at Kingston College, on Saturday, January 21. Anderson's winning throw was 26.89 metres. Second was Immaculate's Lori-Ann Bennett (26.59) and third was Trishawna Williams (24.38 metres) of Vere Technical. - Andrew Smith Photo

1.18 Describe the work of rivers (erosion, transportation, deposition) and the associated features;

1.19 Examine the relationship between the features formed and the processes and factors operating within the river valley.

My method of reviewing the topic is by answering questions on it, taken from past examination papers. Here are two questions on the formation of river features.

June 1998 ­ Question 2

(d) Explain how any TWO conditions
contribute to the formation of deltas (8 marks)

June 2000 ­ Question 3

(e) (i) Explain how levees are formed. (4 marks)

(ii) Explain how the development of levees may result in the development of deferred junctions. (2 marks)

Before answering these questions, let me share with you, something about the work of the river. As indicated in the specific objective 1:18 above, the geologic work of streams consists of three closely interrelated activities:

(i) It dissolves and erodes the land surface over which it runs;
(ii) It transports the matter it has dissolved and eroded away; and
(iii) It deposits the material which it has carried in suspension or rolled along.

Different features are formed as a result of the work of the river. The features you are being tested on in the questions above, namely deltas and levees, are both formed as a result of deposition by the river. How does deposition by the river occur? Let us be reminded of that.

Deposition of material takes place more or less gradually over the whole lower course of a river but it is more concentrated where the speed of the current is abruptly checked for some reason.

This may occur

(i) By a sudden change in gradient

(ii) Where its constricting narrow valley suddenly opens out into the broad gentle floor of the main valley

(iii) By the entry of the river into the still waters of a lake

(iv) By its entering the sea.

Let us now focus a while on deltas. As a further bit of background information for you, the Dictionary of Geography defines a delta as 'A flat area of alluvium at the mouth of a river, where its speed is checked, its load deposited, and the fine particles in the load aggregate settle upon meeting salty sea water. It is usually traversed by its distributaries.' There are different types of deltas, namely:

(i) ARCUATE ­ a very common type is composed of coarse sediments such as gravel and sand and is triangular in shape.

(ii) BIRD'S FOOT ­ is composed of very fine sediments called silt.

(iii) LACUSTRINE ­ may be formed where a river enters a lake.

(iv) ESTUARINE ­ develops in the mouth of a submerged river.

Conditions that are apt to produce deltas may be related to the characteristics of the river, the drainage basin, or to the conditions in the body of water into which the river flows. To build a delta, the best combination of river and drainage basin conditions would be:

1. A heavy load, and this will happen if there is active erosion in the upper section of its valley;

2. Much soft sedimentary rock within the drainage basin to provide the load;

3. A fairly slow-moving stream so that the load could not be carried out too far;

4. The load must be deposited faster than it can be removed by the action of currents and tides.

Conditions in the bodies of water into which rivers flow that encourage deltas are:

(a) Comparatively shallow water at the river mouth;

(b) Weak or non-existent coastal currents so that the sediment is not carried away; and

(c) Small tidal ranges, since a large tidal range has a flushing effect.

I will discuss the other question on levees in the next lesson.