A lake is an accumulation of water either in a depression or in a large basin.  The depression may vary in since from a few square metres to thousands of square kilometers.   Lakes are usually in land directly joined to a sea or ocean.

CLASSIFICATION OF LAKES

Lakes are classified according to their mode of formation, in other words, how the hollow or depression originated. Sometimes  lakes may be classified according to the amount of salt they contain, that is, whether they are salty or fresh water lakes.  The following classification is the most common.

1.     Lakes produced by erosion

2.     Lakes produced by glacial erosion

3.     Lakes produced by earth movement

4.     Lakes produced by volcanic activity

5.     Lakes produced by man's activity.

Lakes produced by Erosion.  There are three categories of lakes produced by erosion, namely those produced by glacial erosion, solution and wind erosion.

Lakes produced by glacial erosion - glacial lakes.  These lakes are less common in East Africa because glaciers cover only a small part of the higher mountains.  One of the commonest glacial lakes is that found in a cirque/curie.  Ice scoops out this armchair shaped hollow high up a glaciated mountain.  When the ice melts water collects in the depression to form argue lake, which is often called a tarn.  Examples of these lakes include:  Teleki and Homily tarns on mount Kenya,  Lac du Speke and Lac Catherine on Mt. Ruwenzori.

Another type of lake formed by glacial erosion is known as Trough lake.  It occupies an elongated hollow excavated by ice on the floor of a U-shaped valley.  It is formed when glaciers erode the bottom of a U-shaped valley unevenly.  It may be that these can be eroded more easily.  As a result the glacier depends that part of the valley more than the rest of the valley, leaving a finger like depression, which can be filled with water after the ice, has melted.  Such a lake is called a finger lake/ribbon lake/trough lake as shown below.

 

 

 

There is also Rock basin lakes formed by the scouring action of an ice sheet or a valley glacier.  These lakes are very common on the Baltic and Canadian Shield.

Lakes produced by wind erosion.  Wind deflation sometimes produces extensive depressions, which reach down to the water table in arid areas.  The lakes of these depressions are not always true lakes - they may be nothing more than muddy swamps.  The Qattara Depression in Egypt, is a good example.

More permanent desert lakes develop when an aquifer becomes exposed.  These lakes are called Oases.  These can be seen in a diagram below:

Some desert lakes dry up because of excessive Oasis evaporation and all that remains is a lakebed of salt.  This is called a playa or a salt lake.

Lakes produced by solution.  Limestone is a rock, which dissolves in acidic water.  The solution of limestone rock may create a depression, which may be filled with water to form a lake.  There are several lakes in Africa, which have been formed in this way, e.g. Lake Ojikoto in Namibia.

Lakes produced by deposition.  Deposition of materials may lead to damming of a river.  Materials such as sediments or alluvium may be deposited to form a kind of barrier behind the barrier a lake may be formed.  There are three types of lakes produced by deposition, namely river deposition, wave deposition and glacial deposition.

 

Lakes produced by river deposition.

(a) Oxbow lake.  It is formed when a meander of a river on a flood plain is cut-off by deposition of alluvium as shown in the diagram below.

Example of ox-bow lakes include Lake Gambi and Lake Shakababu on the Lower Tana.  On River Yala there are Lake Nyamboyo and Lake Kanyaboli etc.

 

(b) Delta lake.  This is formed by the deposition of alluvium by rivers turning either a part of the sea into a lagoon, or part of a distributaries into a lake.  The Nile Delta in Egypt contains a number of such lakes, with others in the making.

(c ) Flood plain lake.  A lake sometimes prevents water from returning to the river, thus causing a lake to form.  There are several lakes of this type on the River Congo in Zaire.  Lake Matohi is an example.

Lakes produced by ice deposition:

(a) Moraine-dammed lake.  If a terminal Moraine is deposited across a river valley it could block the river and thereby create a dammed lake as shown in the diagram below.

Many glacial lakes occupy basins formed by the damming of valley by the terminal moraine ridge.  On Mt. Kenya Tyndall Tarn at the head of Teleki valley is a moraine-dammed lake.  Others include lake Hohuel and Hut Tarn on Mt. Kenya.  On Mt. Rwenzori they include Lac Cris and Lake Mohama.

(b) Boulder-lay lake.  On the lowland of glaciated regions, boulder clay deposits may create depressions, which might e occupied by melt-water to form small lakes.  These lakes are called Kettle Lakes.

Lakes produce by Moraine deposition.  In areas where there is a strong prevailing wind, waves may move material along the beach by the process called longshore drift.  This may reach the other side of a bay and form a bar.  This bar will block off the water in the bay to form a lagoon or lake.  Lake Nabugabo  in Uganda was formed in this way.  Lake Victoria was at one time at a higher level.  At that time alongshore drift from the South formed a bar.  This bar grows northwestwards across the mouth of a bay.  The body of water, which the bar enclosed, is now called Lake Nabugabo. The formation of Lake Nabugabo on the Western edge of Lake Victoria)

 

Lakes produced by volcancity.  Lakes produced by volcanic activity are of two types, namely crater lakes and lava-dammed lakes.

Crater lakes.  Some volcanoes particularly extinct volcanoes may have their tops blown off when a volcano erupts a second time; or the top of the volcanic cone may collapse when the eruption stops.  In both cases a crater or calderas is formed.  These craters may be occupied by rainwater to form lakes.  Generally these lakes are circular in shape.  Lake Shalta in Ethiopia is the largest Crater Lake in Africa.  Other examples include Lake Ngozien Tanzania.

 

 

 

A volcanic eruption may happen when gases build up such a pressure underground that they blow a hole in the ground as they escape.  They may end up with steep sides and a flat bottom in which rain water collects to form a lake.  Examples are those lakes in South West Uganda like Katwe, Nyungu etc.

Lava dammed lakes.  A flow of lava from a volcano may sometimes block a river valley leading to the formation of a lake.  The local drainage pattern may be affected by the solidification of lava on the surface.  Natural dams are formed when the solidifying lava blocks rivers and streams.  A famous example of such a lake is provided by lake Kivu in the Western rift valley and L. Bunyonyi in Kigezi in South Western Uganda.

Lakes produced by earth movements.  Lakes formed/produced by earth movements fall into two categories, namely crystal warping and faulting.

(a) Crustal warping.  The lakes under these categories include:  Lake Victoria and Lake Kyoga.  They were formed by down warping followed by up warping in the West, which caused rivers like Kagera, Katonga and Kafu to reverse their direction of flow.  The following stages of formation can be analyzed:

 

 

 

 

Before the formation of these lakes, the relief of Uganda was gently tilted towards the West, and the river like Katonga, Kagera and Kafu were flowing westwards as shown in the diagram below:

A sketch map of Uganda showing  the direction of flow of rivers before up warping and down warping.

 

 

 

 

 

 

 

 

 

 

 

 

 

During the formation of the basins, Uganda was subjected to compressional and tensional forces/Earth movements, which culminated into up warping and down warping.  The western and eastern parts of the country were up warped while the central part was down warped.  This led to creation of saucer shaped depressions (the Victoria and Kyoga basins), with a gentle tilt towards the north and center.  This can be seen in the diagram below:

 

 

 

 

 

 

 

 

 

The up warping of the area around the basin, culminated into reversal of drainage of the rivers, Katonga, Kafu and Kagera on the Western side.  The waters of these rivers flowed into the depressions through back ponding to produce the lakes as shown below:

 

A sketch map of Uganda, showing the reversed rivers after up warping and down warping.

 

 

 

 

 

 

 

 

 

The backponding of Lake Victoria gave chance to the water escaping northwards at the ripon/Nalubale falls along the Victoria Nile leading to formation of a large swamp which is today L. Kyoga and Kwania.

The rejuvenation of the western rivers like Nzoia, Mara, Nyando etc also contributed to the filling of the basin of L. Victoria leading to overflow of Lake Victoria.

Heavy rainfall during the Pleistocene period also contributed to the filling of the basins.

Other examples of lakes due to the reversal of rivers like Katonga and Kafu are Bisinia, Wamala, Mburo, Kijanebalola, Kachira, Nakkwali etc.

 

Characteristics of depression lakes:

(a) Rift valley lakes (Lake produced by faulting).  These lakes lie on the floor of the great East African rift valley.  They include Lake Turkan, Lake Basingo, Lake Bogoria, Lake Nakuru, Lake Naivasha, Lake Elementaita and Lake Magadi, all in Kenya.  In Tanzania they include Lake Tanganyika (second lake in the world, 1470m), Lake Manyara, Lake Eyasi and Lake Rukwa.  In Uganda they include Lake Albert, Lake George and Lake Edward.  Lake Malawi on the border between Malawi and Tanzania is also a lake due to faulting.

These lakes were formed due to earth movements that culminated into faulting by tensional or compressional forces.  These forces led to the formation of an elongated trough (rift valley) as shown in the diagrams below:

 

 

 

 

 

 

 

Formation of the rift valley by tensional forces.

Tensional forces pull apart two masses either sides of a central block so that the center is allowed to subside as a trough.  The main bounding  faults are normal as shown below.

 

 

 

 

 

 

                             OR

Formation of Rift valley by compressional forces.

 

 

 

 

 

 

 

Compressional forces push together two masses either side of a central block, causing them to thrust up and over-ride the center, holding it down as a trough.  The bounding faults are reversed.

 

After the formation of the rift valley, there was multiple (secondary) faulting which resulted into formation of a depression/garden within the rift valley as shown below:

 

OR

Lakes produced by man (man-made lake).  This is often called a reservoir.  It is deliberately formed by building a dam across a narrow, steep-sided section of a river valley, usually a gorge, for the purpose of storing water for irrigation, or for developing hydroelectricity or both.  Examples of man-made lakes in East Africa of this kind include Masinga Dam, Luitaru Dam, and Kamburu Dam, all in Kenya,  Pangani Dam in Tanzania and Nalubale Dam, in Uganda.

Lakes due to landslides.  The occurrence of a landslide may block a river valley.  Thus water accumulates to form a lake.  There are several lakes in Africa formed in this manner.  They include Lake Buguku in the Ruwenzori Mountains and Lake Nyabihoko in South West Uganda.

 

 

 

The Contribution of Lakes To East Africa

Positive Contribution:

Lakes are sources of fish, which serve: in the first place as a rich animal protein for people. Secondly, as a source of raw materials for industries e.g. Masese Fish Canning factory in Jinja, Uganda Fish Exporters Ltd. (UFEL) etc, thirdly, as a source of income for the local population and exported to gain foreign exchange; and finally as a provider of employment to people.

Lakes facilitate water transport for inland and international connections.  Internally there were steamer services from (Nimule in Sudan) to Pakwach and onto Butiaba on Lake Albert and from Masindi Port to Namasagali on Lake Kiyoga in Uganda.  There are also international connections on Lake Victoria like Port Bell to Mwanza and Port Bell to Kisumu etc.

Lakes have facilitated the growth of Ports/Landing sites/fishing villages.  Ports like Port Bell, Jinja, Majanji etc in Uganda Kisumu in Kenya and Mwanza, Musoma etc in Tanzania on L. Victoria, Kigoma, Nkasi etc on L. Tanganyika in Tanzania etc.

Lakes provide constant water supply (act as Reservoirs) for production of electricity e.g. L. Victoria ensures constant water supply for the Nalubale power project at Jinja in Uganda.         

Lakes may be used to supply water for irrigation purpose e.g. water from Lake Victoria is used for irrigation at Kakira Sugar Works in Uganda.  Water from Lake Naivasha is also used for irrigation purposes.  Besides water from Lake Victoria is used for irrigation in the Yala and Kano areas in Kenya.

Lakes act as tourist attractions e.g. Lake Victoria the third largest lake in the world; Lake Mburo in Uganda is surrounded by a National Park, Lake Nakuru in Kenya is part of the Lake Nakuru etc. Lakes have good beaches like Gaba, Resort etc along L. Victoria and sports activities like sport fishing, swimming etc in lakes like Victoria for both Uganda and Kenya; and Turkana in Kenya which have promoted recreation and relaxation.

Lakes provide water for industrial and domestic use e.g. Lake Victoria is used by industries in and around Jinja and Kampala in Uganda.  Also other industrial centers like those at Kisumu  in Kenya use water from Lake Victoria.  At the same time the inhabitants of the towns and villages around the lakes use their waters for domestic purposes.

Some of these Lakes contain valuable minerals that are being exploited to generate revenue.  For example Lake Katwe contain salt.  Soder ash or troria is mined at Lake Magadi in Kenya. Besides the shores of these lakes have sand and clay which is extracted for construction.

Lakes have encouraged international and internal trade and international relationship.  For example there is  lucrative border  trade between Uganda and DRC along Lake Albert.  There is good understanding among the three East African countries because they share Lake Victoria.

The shores of some of these Lakes have papyrus swamps and palms, which provide raw materials for art and craft.  For example the papyrus swamps at Kajjansi along Lake Victoria are being used for this purpose etc.

Large Lakes may modify the climate of the surrounding regions - through land and Lake breezes, evaporation and condensation to cause convectional rainfall.

Some lakes together with their swamps especially those in central Uganda are very important because during peak rainfall, they act as sponges thereby absorbing excessive water and thus reducing dangers of flooding e.g. Lake Victoria.  They also regulate the flow of rivers.

Lakes in East Africa have promoted educational research into water resource use.  For example the Fisheries Research Institute (FIRI) at Jinja in Uganda is responsible for research in the Fisheries resources in Uganda. e.g. Lake Victoria, Kyoga Albert etc.  Besides students at different levels go for fieldwork research studies.

NEGATIVE CONTRIBUTION

Lakes have also contributed negatively to the development of Uganda.  These include the following.

They occupy much land, which would have been used for other activities like agriculture.  For example, Uganda's lakes covers nearly a fifth (48,300 km2) of its area.  Lake Victoria alone covers an area of 28,655 km2 which areas would be useful for agriculture and settlement.

Wild animals like crocodiles, snakes etc which are dangerous o human live in  these lakes for example  lakes like L.Victoria and Kyoga have crocodiles while snakes are in every lake.

Accidents by storms/drowning do occur on the lakes.  For example about 30 people drowned on L.Victoria in March, 2001 when traveling to Kasenyi.

The presence of the water weed (hyacinth) which caused a  problem to the utilization of the water resources in Uganda.  For example, water hyacinth mats have closed important leand sites/ports like Kibanga on Lake Victoria in Uganda.

Lakes and the swamps surrounding them are breeding grounds for disease carrying pests/vectors like mosquitoes which cause malaria, snails which cause bilharzias etc.  This is common along the shores of Lake Victoria, Kyoga etc in Uganda.

Lakes encourage smuggling/pirates since it is hard to monitor the activities going on, on these lakes.  Smuggling is common on border lakes like Albert, Victoria, Tanganyika etc.

Lakes encourage flooding due to water level fluctuations.  For example lake Kyoga flooded in 1999 and destroyed a lot of property and crops.

Lakes facilitate the spread of pollutants e.g. Lake Victoria which is near major industries in Uganda.

Lakes encourage border conflicts e.g. Albert and Victoria etc.

 

Revision questions

1.To what extent are the lakes in East Africa a result of tectonic movements?

-Define tectonic movement and bring out the causes of the movement.

-Identify the various forms of tectonic movement e.g.

-Identify the lakes resulting from these tectonic movements e.g.

-Identify other processes of lake formation e.g.

 

2.       To what extent are erosion and deposition responsible for the formation of lakes in East Africa?

-Identify the lakes formed by the processes of erosion e.g.

  Glacial erosional lakes - cirgue lakes

  Solution lakes - removal of certain rocks in solution to provide hollows like

  those  of limestone region e.g. in Nanyuki in Kenya.

  Wind erosion lakes through the process of deflation e.g. Oasis lakes.

-Identify the lakes formed by deposition e.g.

-Identify other processes responsible for lakes formation in East Africa e.g.