Description of drainage
Marjorie Henry, Contributor

Greetings to you all, my young readers! I trust that you are getting along well with your studies, especially since your external examinations are inching closer and closer. For several weeks now I have been reviewing some practical skills you are required to have in preparation for the examinations. Those I did were - stating directions both by using the compass points and bearings; measurements; grid references both four-figure and six-figure; gradients; cross section and the use of contours to identify landforms. I will end these series of lessons with descriptions. In reference to the syllabus, it is stated in the specific objective 1.8 that you should be able to:

Describe the following: drainage, vegetation, land use, settlement and communications.

This week I will begin with the description of drainage.

On the ordnance survey maps, for example, the one you are given to use in the examination, drainage is represented in blue. Get a hold of any available ordnance map extract and read what is written in the legend or key with regard to how drainage is shown on it. For example, you may see single blue lines representing watercourses; enclosed blue lines with the enclosed area also shaded in blue, representing ponds or lakes; swamps, dams, wells and springs are also shown. It is important that you observe carefully what is included in the legend in order for you to interpret the map correctly.

In describing the drainage, here are some questions you should ask, the answers for which will help you to focus on the details needed for a proper answer.

(a) How many rivers are there?

This requires relative terms, namely, many or few. You could write, for example, 'There are many rivers in the north of the map but few in the south.' You could also state - 'The northern section of the map is well-drained. Many rivers are found in that area.'

(b) Are there areas of poor drainage?

Look for areas of swamp on the map. Again, the legend will guide you. State the location of the swamp using the grid lines.

(c) How wide are the rivers?

It is not mandatory that you do any measurement. This can be determined by the width of the blue line that represents the rivers. A small stream is shown by a single blue line, while a larger one is shown by two blue lines enclosing blue shading. The small stream can be described as 'narrow'.

(d) What type of streams are they?

The blue lines representing the streams can either be solid or detached lines. The solid lines indicate that the rivers are perennial, that is, water is in the channel all through the year. In contrast, the detached lines indicate that the streams are seasonal, flowing only sometimes during periods of heavy rainfall. Here is a small section of a map of Antigua showing seasonal streams. The detached blue lines are evident throughout.

In discussing the type of streams you could also indicate if the streams disappear. Study the map carefully to see whether the streams flow to the sea or other water bodies. Where they do not and appear to be somewhat 'isolated', these suggest that the streams disappear. Disappearing streams also inform you about the nature of the rock. They are typical of limestone rocks.

(e) What is their relative speed?

You are trying to determine if the rivers are fast or slow flowing. This can be judged by the contour spacing. As indicated in an earlier lesson, when the contours are closely spaced, a steep slope is present. In contrast, when the contours are widely spaced, a gentle slope is present. A river flowing over a steep slope is usually fast flowing while one over a gentle slope is slow flowing. A fall in height from one contour to another can also guide you to determine the speed of the river.

(f) Are they straight or meandering?

For this, simply study the map to see how the blue lines are drawn. Are they relatively straight lines or are they twisting and turning? There is some relationship between this and the relief of the land as meandering streams are usually found in low-lying areas and relatively straight streams in highland areas on steep slopes.

The discussion on the description of drainage will continue in the next lesson.

Marjorie Henry is an independent contributor. Send questions and comments to kerry-ann.hepburn@gleanerjm.com