To what extent has relief influenced the development of drainage patterns in East Africa?

Candidates should define the term drainage pattern

Drainage pattern is s the layout/plan or arrangement made by a river and its tributaries on the landscape (Drainage basin) over which it flows.

Candidates should identify and describe the various drainage patterns that exist in East Africa as:

• Radial drainage pattern – A pattern in which rivers and their tributaries flow from a common peak/dome shaped structure  various directions like the spokes of a bicycle wheel. It is common in dome shaped uplands like mountain Elgon, Mgahinga etc.
• Dendritic drainage pattern – a pattern where rivers and their tributaries create a tree like plan/layout, with tributaries joining the main stream from many directions at more or less acute angles. It’s the most, common type of drainage pattern in East Africa, such as river Apwac in Kalongo area, Tana river, rive: Namatale etc.
• Rectangular drainage pattern (Trellis) – a pattern where the main stream takes sharp, more or less rectangular bends and tributaries join it at more or less right angles such as river pager and tributaries, river Athi, river Mayanja, river Kato-Wasswa in Mityana-Mubende area etc.
• Centripetal pattern – where rivers flow from the rims of surrounding higher areas (Basin) into a common depression where there is a lake or Swamp such as lake Victoria and Kyoga, lake Baringo etc.
• Annular pattern – where streams join at sharp angles arranged in a series of curves around dissected uplands, craters / calderas such as Ngorongoro, Bukigai area (hill) in Bududa etc.
• Parallel pattern – where streams and their tributaries flow down slope more or less parallel to each other such as nver Nkusi and Hoima on the other Butiaba scarp, river Rukoki, chalanga and Kamulikwizi in Kassese area.
• Barbed/ hooked pattern – a pattern where stream tributaries flow in opposite direction to the main river, before joining it at more or less acute angles to form a hooked or barbed pattern. This pattern is associated with drainage reversal as is the case of river Katonga, Kafu, Kagcra etc.

Candidates should explain the extent to which relief has led to the development of the above mentioned drainage patterns in East Africa as:

The existence of relief has influenced the development of various drainage patterns in many ways and these include:

• Steep slopes on volcanic cones such as Mt Susisa in Kenya, Mt Elgon etc. favor the development of radial drainage pattern. The steep slopes accelerate down ward movement of water and erosion of rocks to create channels along which rivers flow. Steep slopes such as escarpments lead to parallel drainage patterns.
• Gently dipping slopes favor the development of dendritic pattern. Gently sloping areas encourage the dendritic drainage where the consequent (major) and subsequent (minor) streams flow in the direction of the initial slope over which the pattern was established such as river Malagarasi Victoria Nile, Rufiji etc.
• Existence of hills separated by inside valleys lead to the development of trellised drainage as seen from Mayanja Kato and Wasswa in Mityana-Mubende area.

Candidates should  explain the other factors which equally influence the development of drainage Patterns in East Africa such as:

Rock nature/ structure

• Jointed/ faulted rocks have encouraged the development of rectilinear/ trellis drainage patterns as seen on river Mayanja-Kato and Wasswa, pager, Awa and Galana.
• Alternate soft and hard rocks demarcated by joints almost at right angles to the general slope encourage trellis drainage patterns.
• Soft and hard rocks lying side by side encourage the development of parallel drainage pattern, where rivers flow by side of each other but with limited chances of joining e.g. river Nkusi and river Hoima.
• Homogeneously uniform/crystalline igneous rocks lead to the development of dendritic patterns, radial patterns. Uniform rocks enable the rivers to erode uniformly creating a variety of tributaries.

Tectonism has encouraged me development of drainage patterns in East Africa;

• Warping- Areas affected by up warping and down warping such as the Victoria-Kyoga basin/depression, which later encouraged several rivers from different directions to flow into the basin, formed centripetal, drainage pattern.

• Faulting encouraged the formations of joints and faultlines which later promoted the formation of rectilinear/rectangular or parallel patterns.
• River capture system encourages the development of drainage pattern over time especially where a strong river arrests the water of a weak neighboring river, into its own channels. This encourages the development of barbed/ hooked drainage patterns, Dendritic patterns etc.
• Climate in form of reliable rainfall equally accounts for the development of drainage patterns in East Africa. Existence of reliable rainfall in a drainage basin/catchment area is necessary to support the evolution and continued existence of a river and its tributaries which may form several patterns like radial, trellis, dendritic etc.

Much as relief plays a significant role in a drainage pattern development, it does not do so in isolation, other factors are also at work.

N.B: Candidates should draw diagrams to illustrate their answers where appropriate and local examples may be sighted to support their answers.