How to describe drainage in topographical map

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what is drainage

Drainage is the outflow of water through the system of natural rivers

what is drainage pattern?

A drainage pattern is a layout that a river and its tributaries, or set of rivers, make on the earth’s surface. Some of the common river drainage patterns include dendritic river pattern, trellised river pattern, radial river pattern and centripetal river pattern

The following are hints to follow when you describe drainage in topographical map

Divide the map into drainage basins using compass directions. Note that the drainage basin is an area drained by single river system.
In each drainage basin explain the drainage pattern which the river system in the basin take. The common river drainage pattern are

For each drainage basin explain how the rivers drain the relief region i.e highlands, lowlands and plateaus. at this part state the direction of the flow of the river, where the river is pouring its water and whether the area is well drained or not. The area is well drained when there is no swamps or marshlands and is not well drained when is waterlogged areas.

Dendritic drainage pattern

dendritic drainage pattern require the following condition to be formed:

rock should be uniformly resistant to erosion
this pattern is not dependent on geology

characteristics of dendritic drainage pattern

the pattern is tree shaped
the tributaries join the main stream at acute angles

example of rivers with dendritic pattern include tigela, mooi and umgeni in the south Africa

Radial drainage pattern

radial drainage pattern is formed on dome shaped hill, conical hill, mesa or butte

characteristic of radial drainage pattern

streams radiate outwards from the central high point.

example of areas with radial drainage pattern includekoffiebus and teebus in south africa

Trellis drainage pattern

requirement for trellis drainage pattern to be formed

parallell ranges of fold mountains or alternate layers of hard and soft rock
a strong main stream
the pattern is dependent on the relief and the geology of the area

charateristics of the trellised drainage pattern

tributaries meet the main stream at the right angles
the main stream cut gaps through the mountains

example of areas with trellis drainage pattern include beffels river at laingsburg in south africa

Centripetal drainage pattern

this type of drainage pattern is formed on a central basin or a low lying area such as a lake or a marsh

characteristics of centripetal drainage pattern include:

streams radiate inward from a sorrounding higher area
it is opposite of radial drainage pattern

example of centripetal drainage pattern is flatter parts of namaqualand in south africa

parallel drainage pattern

parallel drainage patterns occur in the recently uplifted sloping plain and require a rock type that is uniformly resistant to erosion

characteristics of parallel drainage pattern

the main streams flows parallel (or nearly) to each other
the tributaries enters the main stream at very small angles

An example of an area with a parallel drainage pattern is tributaries of the steelport river in Limpopo

ractangular drainage pattern

this occurs in a well-jointed rock with joints exposed at the surface

characteristics of rectangular drainage pattern

rivers flows in exposed joints
rivers and tributaries have right angled bends

An example of areas with rectangular drainage pattern is rivers in Waterberg (Limpopo)

deranged drainage pattern

this type of drainage pattern occur in geologically young areas

characteristics of deranged drainage pattern

haphazard pattern
numerous lakes and swamps
streams enters and leaves swamps and lakes randomly

example of areas with deranged drainage patterns are recently glaciated areas such as the Canadian shield

factors that influence the development of river drainage pattern

The slope of the land

The direction of the river Flow is generally guided by the direction of the slope since the flow of the water is controlled by gravitational force.

There are rivers whose flow is exclusively determined by the direction of the slope.

Difference in rock resistance or hardness

As the river erodes its channel, the nature of the bedrock becomes significant in determining how deep the river channel will become.

If the river encounters a very resistant rock outcrop in its path, it may have to change its direction to circumvent it despite the nature of the slope.

The rock structure along the river’s profile.

At certain stages in the course of the river, The arrangement of rock layers may influence the direction that the river takes.

The river may change its direction due to the direction of the strike of an exposed rock layer despite the general direction of the flow.