Describing drainage on maps
Marjorie Henry, Contributor

Another aspect of map reading is description. You all need to be able to describe drainage, vegetation, land use, settlement, communications and landforms. The last section of question one from the June 2008 paper tests description of drainage. It states:

Compare THREE features of the drainage west of the Rio Grande with the area to the east of it. [Refer to the entire area of the map extract]

Before discussing this, I would like to share with you some general points for the description of drainage.

Recommended approach

The recommended approach to doing a description is to ask yourself some questions. The answers to these must refer to actual areas, features and names on the map you are working with. Grid references must be given for specific examples of points or areas. Here are some questions to guide you through a description of a drainage system (a brief explanation follows each question).

(a) How wide are the rivers?

A blue line is used to represent rivers on the map. If this is a single line, it is indicating a small stream. On the other hand, if there are two blue lines enclosing blue shading, then it is a large river.

(b) From what height does the river begin?

Identify the height of the contour from which the river begins.

(c) What is the general direction of flow?

There is no need to indicate the direction of every twist and turn of the river. As the question suggests, identify the general or main direction that the river is flowing in.

(d) Are the rivers seasonal or perennial?

Refer to the legend or key of the map to identify this. If there is a solid blue line, then it is a perennial stream. However, if the blue line is detached, then it is a seasonal stream.

Disappearing rivers

(e) Is it all surface drainage or are there disappearing rivers?

If a river appears to begin and end 'abruptly' on the map, that is, it does not enter the sea or lake, then that is deemed to be a disappearing river. This information also tells you about the nature of the rock. Disappearing rivers are indicative of limestone rock type. An abundance of surface streams suggests some type of impervious rock.

(f) What is the relative speed of the river?

This is determined by the spacing of the contours. If they are close together, this suggests a steep slope. Usually, a stream on such a slope would be fast-flowing. When the contours are widely spaced, a gentle slope is present and a stream on that slope is likely slow-flowing.

(g) What features can be identified in the channel and course of the river?

Deposition of materials can be present and there can be braiding. Rapids and waterfalls can also be present.

Drainage pattern

(h) Which drainage pattern can be identified?

A radial pattern is usually associated with a dome or conical hill. At the same time, individual streams within this pattern can display another pattern, namely dendritic or trellis.

(i) Can the stage of the river be identified?

Look for the following:

(1) Youth - rapids, deep valleys, steep gradients and a direct course.

(2) Maturity - wider valleys, gentle gradients, strong meanders.

(3) Old age - wider valleys (that is, a less pronounced 'V' shape), flood plain, absence of contours, contorted meanders, oxbow lakes.

Human inteference

(j) Has man affected the drainage pattern?

Look for dams and possibly straightening of the course.

In reference to the section of the question stated earlier, please note that you are asked for a comparison between areas east and west of the Rio Grande. This must be point and point, that is, whatever you say about a feature on one side, you need to say something about the same feature on the other side. In reference to the map extract for the question, you could say, for example, that to the west of the Rio Grande there are numerous surface streams while to the east there are only a few.

Features to be compared

The point here is on the number of streams found. Other features that could be compared include the width of the rivers, their meandering and the height at which they begin.

In your answer, please discuss drainage only. Do NOT discuss other things like natural vegetation, communication line and settlement. The question did not ask for these. It is absolutely necessary that you interpret your questions carefully.

Under the theme 'Eat what you grow, grow what you eat', these Irwin High School agricultural science students show off their display, which was a hit during the annual exposition hosted at the school. - Photo by Denise Reid