ACCORDANT AND DISCORDANT DRAINAGE PATTERN

 Accordant drainage patterns.

This is where the river and its tributaries flow according to the slope of the land the dendritic drainage pattern, annular, radial etc. on the other hand, discordant drainage pattern refers to a situation where there is no systematic relationship between the river and the underlying rock type and structure.

Accordant drainage pattern

  1. Dendritic drainage pattern; dendritic comes from a Greek word Dendron which means tree like. This is a tree-like pattern of drainage in which tributaries’ coverage on the main stream from many direction and usually join the main river at acute angles i.e. less than 90 degrees. It develops in gently dipping areas which influence the speed of river flow.

Conditions under which it develops

  • It usually develops in areas of homogenous rocks with the same resistance to erosion e.g. granite

  • It also develops in areas of gently sloping land

  • It develops in areas of heavy precipitation or rainfall.

  • All rivers and tributaries flow in valleys that are proportioned to their size.

  • The tributaries flow and join the main stream through head ward erosion eg R. Nzoia in kenya, river Athi, Nyendo in kenya, opethi and moroto in Uganda.

  • Rectangular or trellis drainage pattern; this is the drainage pattern where the tributaries join the main river at roughly right angles. Its formation is attributed to the following conditions;

  • Nature of the rocks. Trellis drainage pattern occurs in areas with alternating belts of hard rocks which lie at right angles to the general slope. So the river and its tributaries flow only on the soft rocks and if at all tributaries are to join the main river, its only possible at right angles.

  • Tectonic movements; trellis drainage patterns can also occur in faulted regions where the river will take the shuttered rocks of the fault line as soft rocks hence it will flow along fault lines thus joining at right angles.

  • Climate; trellis drainage patterns take place in areas with heavy and reliable rainfall in order to maintain constant volume of water in river and streams.

Examples of trellis drainage patterns include or can be found on rivers Mayanja, kato, wasswa etc in Uganda

  1. Radial drainage pattern

It’s a drainage pattern where the rivers flow outwards in all directions from a central point which is usually a dome or cone shaped volcano.

Conditions for the formation of a radial drainage pattern.

  • Develops in areas with a dome shaped volcano

  • The river flows in accordance with the slope of the land downwards in all directions.

  • Develops in areas with homogenous rocks

  • Develops in areas with heavy precipitation or rainfall eg mt. elgon, Rwenzori in Uganda, Kilimanjaro and Kenya.

  • Centripetal drainage pattern

It’s a drainage pattern where the rivers flow inwards towards a depression which is usually a lake.

Conditions.

  • Develops in areas of depression and the rivers flow down into it from all directions.

  • Develops in areas with heavy rainfall.

  • The rivers follow the slope of the land

  • They flow in areas with homogenous rocks eg rivers flowing into Lake Victoria and Baringo in Kenya; they include Loboi and Olmukutan.

  • Parallel pattern

It’s a pattern where the river flow parallel to each other from the same watershed which can be an escarpment.

Conditions

  • It develops in areas with steep elongated slopes such as escarpments.

  • Develops in areas of homogenous rocks

  • Develops in areas with heavy rainfall

  • The rivers follow the slope of the land downwards eg rivers flowing down the western edge of the Butiaba escarpment.

  • Hooked or barbed pattern

In this pattern, the tributaries appear to flow in the opposite direction to the main river and look like a hook or a barb. They enter the main river at an acute angle.

Illustration.

  • Usually develop in areas of river capture where drainage reversal occurs.

  • Develop in areas of heavy rainfall eg river kafu, katonga and kagera in Uganda and the Rwizi river.

This pattern resembles a feather. In this case, tributaries join the main river at acute angles to make the pattern look like a feather

  • Annular pattern

It’s a circular pattern than develops within a volcanic crater. The streams join at sharp angles and are arranged in series of curves in the crater. It’s not a common pattern and examples can be seen at lake Basumtwi in Ghana.

DISCORDANT PATTERNS

A discordant pattern refers to where there is no close relation between the rocks and the drainage pattern exhibited by rivers. The major discordant drainage patterns include;

Antecedent drainage pattern.

It involves a river which developed and established a pattern on landscape which was later uplifted by earth movements. The river conditions to flow in its original direction by eroding a gorge in the landscape.

The uplift must be slow enough for the river to maintain its course in the path of the rising ground or land. The river must also be powerful enough to erode vertically downwards as fast as the rising land and therefore develops a gorge. Antecedent river patterns are therefore older than the landscape over which they flow eg R. Malagarasi on Tanzania.

Super imposed drainage pattern.

This is the pattern that develops on a landscape which is afterwards is removed or changed. It’s now super imposed or on top of a previously buried and completely different rock structure eg if sedimentary rocks are covering a folded rock structure as shown below.

When the sedimentary rocks are removed by erosion, the folded rocks below are exposed. The river is normally expected to flow through the synclines or valleys. However, it may manage to maintain its course and cut across the anticlines by developing gorges. Super imposed drainage is therefore younger than the landscape over which it flows.

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