Explain the influence of rock structure on the development of drainage patterns in East Africa.

Candidates should briefly describe rock structure as entailing aspects of rock hardness, rock jointing, rock dip, stratification, mineralogical composition, homogeneity, rock heterogeneity etc.

The nature of the rock structure has influenced development of different drainage patterns in many ways. These include:

• Jointed and faulted rocks have influenced development of rectilinear or Trellis drainage This is a pattern which displays a rectilinear shape with tributaries joining, the main stream at approximately right angles (90°). It develops in areas of alternate soft and hard rocks demarcated by joints and lie at approximately right angles to  the general slope downwards which the main river (Consequent) stream flows. The soft rock bands are eroded by head ward erosion while the hard rocks resist. Presence of joints along the rocks facilitates quick head ward erosion in that the river flow is guided by the fault lines or joints.

Examples occur on R. Mayanja- Kato, Wasswa in Kichwamba, R. Aworanga, Pager, Aswa, Tiva and Galana,

• Alternate bands of soft and hard rocks side by side influence development of Parallel drainage pattern,  This is a drainage pattern where rivers flow by the side of each other but with limited chances of joining one another hence the name Examples are found in western Uganda where R. Nkusi and R. Hoima flow parallel to each other before joining L. Albert.  In  Kenya they are found in the Aberdare ranges and west of Mau ranges where tributaries of River Athi such as R. Nairobi, Thirika, Komu and Ruiru flow parallel to each other. Soft rocks are eroded to form the river channel while the hard rocks resist erosion to form a divide which limit chances of adjacent rivers joining each other.

• Homogeneous ot crystalline igneous rocks influence development of dendritic and radial patterns. Dendritic is a drainage pattern where the main river and its tributariesresemble a structure of a tree or a leaf. It’s a tree-like pattern where tributaries converge on the main stream from many directions and usually join the main river at anacuteanglei.e.lessthan90°.

Example are found on R. Rufigi, Victoria Nile, R. Malagalasi. Ruvuma in Tanzania and River Nyando. They develop on areas with uniform rock/structure and hardness {Homogenous rocks). Uniformity of the rock structure enables the rivers to erode any region with minimum effort thus creating a variety of tributaries.

• Difference in rock hardness (Heterogeneous rocks) occurring in alternate belts of hard and soft rocks lying at right angle to  the  slope  influence  development  of  Trellis  This is pattern which displays a rectilinear shape with tributaries joining the main stream at approximately right angles (90°). Examples occurs on R. Mayanja-  Kato,  Wasswa in  Kichwamba. R. Aworanga & Pager. The soft rocks are  eroded through head ward erosion to create river channels while the bard rocks resist erosion resulting into trellis pattern.

• Steeply dipping rock encourage development of radial drainage pattern especially if it is on a dome shaped upland e.g. on Mt. Suswa in Kenya.