Southern Eastern Rift



ID

566

Author(s)

Ashley Brown and Robin Abell, WWF-US, Conservation Science Program, Washington, DC, USA<

Countries

Kenya
Tanzania

Reviewer(s)

Samuel Matagi, WWF, Eastern Africa Regional Program Office, Nairobi, Kenya

Major Habitat Type

Xeric freshwaters and endorheic (closed) basins

Main rivers to other water bodies

Shallow lakes, rivers and streams, hot and cold water springs, marshes, swamps, and salt pans occur within the ecoregion. There are also several artificial wetlands, such as dams, fish ponds, sewage lagoons and irrigated fields.  The major freshwater lakes from north to south are Baringo (130 km2) and Naivasha (156 km2) in Kenya, and Babati, Burungi and Kitangiri in northern Tanzania. The main saline or alkaline lakes are Bogoria (42 km2), Nakuru (49 km2), Elementaita (19 km2), and Magadi (105 km2) in Kenya, and Natron (900 km2), Manyara (470 km2), Eyasi (1050 km2), Barangida and Singidani in Tanzania. There are also other small lakes (< 20 km2) on the valley floor, including Lakes Solai, Kabongo, Kwenia, Sonachi, Oloidien and Ol’bolossat in Kenya; and Bdrangida, Lelu, and Momera in Tanzania. These lakes are salty and some of them have high concentrations of fluoride and sodium carbonate salts.  The lakes lie in the primarily evergreen bushland portion of the Eastern Rift Valley. 

Saline and soda lakes are more abundant in the Eastern Rift than in any other African ecoregion. Streams that feed the lakes flow over highly alkaline volcanic rocks, bringing natron (a naturally occurring salt consisting of sodium carbonate and sodium bicarbonate) into the lakes’ waters (Cole 1994). Many of these lakes are endorheic, and high ambient temperature in the rift valley increases the rate of evaporation, thereby enhancing the water’s alkalinity by raising the concentrations of Na , HCO3-, and CO3 2-. Whereas the pH of freshwater lakes ranges from 6 to 8, that of the soda lakes ranges from 9 to 12.  Many of the soda lakes fluctuate in size and change in water salinity with dry and wet periods. These soda lakes vary in shape from broad, shallow pans to narrow, deep depressions (Livingstone & Melack 1984).

In the Tanzanian rift valley, the largest saline and soda lakes are Lakes Eyasi, Manyara, and Natron. The lakebed of Lake Eyasi is usually dry, but may occasionally flood up to a depth of 1 m (Hughes & Hughes 1992). The Sibiti River occasionally flows into the lake but its waters usually evaporate before reaching the dry lakebed. Even though permanent springs lie along the lakeshores, their waters evaporate quickly. To the east of Lake Eyasi lies Lake Manyara. Manyara is also shallow, with a maximum depth of 3.7 m (Hughes & Hughes 1992). Lake Natron lies to the north on the border between Kenya and Tanzania. Seasonal streams drain into Lake Natron from the Ngorongoro Highlands south of the lake, Mt. Lengai (2,942 m asl) in the southeast, and the Nkito Hills in the west (Hughes & Hughes 1992).The Ewaso Ngiro River, which is the principal affluent, flows through the Ngare Ngiro Swamp or Shompole swamp before entering the lake.  About 28 springs, most of them saline, also feed into Lake Natron. Despite these inflows, most of the lake’s water is derived from direct precipitation. Evaporation exceeds precipitation, and the maximum depth of Lake Natron is only 2 m. A large portion of the lake’s bed is covered by a salt crust that dissolves during the rainy season (Wetlands International 2002).

The northern soda lakes, such as Lakes Magadi, Elmenteita, Nakuru, and Bogoria, are smaller than the southern lakes and tend to fluctuate more in depth and area. For instance, most of Lake Magadi is a dry lakebed, containing water only after heavy rains when water reaches the northern portion of the lake via three wadis (temporary watercourses). The lakebed is composed of trona (solid sodium carbonate) and other associated salts.  Shallow lagoons in the northern and southern ends of the lake are fed by hot springs all year-round, maintaining lake water temperatures at about 40o C (Hughes & Hughes 1992). The strongly alkaline Lake Nakuru typically covers between 0.35 km2 and 0.49 km2 in area but has dried out completely several times over the past 50 years, due to unknown reasons (International Lake Environment Committee 2001). This endorheic lake, with a mean depth of 2.3 meters, is small for its large drainage area of 1,760 km2 (International Lake Environment Committee 2001). The three major rivers feeding the lake are Njoro, Makalia, and Nderit, and it also receives water from several alkaline springs.  In 1998, heavy rains generated by El Niño in Kenya flooded the lake, lowering its salinity (Ramesh Thampy, personal communication). Lake Bogoria, with a mean area of 34 km2 and a mean depth of 5.4 meters, differs from these other lakes by having minimal water level fluctuations (International Lake Environment Committee 2001; Wetlands International 2002). The Wasenges-Sandai River system and numerous springs from the nearby escarpment feed Lake Bogoria while Loboi marshes on the north-western flood plain help to maintain the lake’s hydrology. The lake sits in a graben, which is situated within an area of active faulting, as evidenced by the presence of geysers, steam vents, and hot springs along the lake’s shores (Wetlands International 2002). 

There are also several freshwater lakes within the ecoregion. In the southwest of the ecoregion, Lake Kitangiri is fed by the Wembere River, which flows through an extensive floodplain (105 km long and up to 20 km wide) before it enters the lake from the south (Hughes & Hughes 1992). Lake Naivasha, lies at 1,879 m above sea level and it is the highest of the major rift valley lakes. With a volume of 4.6 km3, it is the largest freshwater reservoir in this ecoregion. There are no known surface outflows from the lake, but scientists believe that underground outflows must exist for the lake to maintain its water fresh (Wetlands International 2002). About 16% of the lake’s waters come from subterranean sources, including seepage of surface flow and inflow from temporary watercourses that drain the slopes of the Ol Doinyo Epuru Ridge. The lake’s water level, which averages 6.5 meters, is known to fluctuate by several meters from year to year.  Lake Baringo, a freshwater lake with an area of 166 km2 and depths recorded between 3 and 8 meters, lies in the north of the ecoregion and contains a large volcanic island in the center as well as several other small islands (Wetlands International 2002). Six rivers drain into the lake from the Mau Escarpment, the Lodiani Mountains, and the Aberdare Range. The main affluent of the lake, the Molo River, flows through extensive swamp belts. Hot springs on the islands and edges of the lake discharge saline water into the lake. There may be an outflow from the lake under the northern lava bed (Hughes & Hughes 1992).



Description

Boundaries

The Southern Eastern Rift ecoregion with its highlands, lowlands, freshwater and saline lakes, and wetlands hosts outstanding congregations of flamingos and several endemic freshwater fish. This rift valley system, which is also known as the Gregory Rift Valley, stretches for over 700 km from central Kenya to northern Tanzania, and covers an area of approximately 3,800 km2 (Hughes & Hughes 1992).

Topography

The valley varies in width from 50 to 100 km and in elevation from 1,850 m above sea level (asl) in central Kenya to 600 m asl in northern Tanzania. The highlands on either side of the valley range in altitude from 2,000 m asl to 3,300 m asl and, especially in Kenya, they are characterized by forest ecosystems, woodlands and open grasslands (often on old volcanoes). 

Freshwater habitats

Wetland vegetation in this ecoregion is highly variable and can be grouped into two categories; flora of saline lakes and wetlands and flora of freshwater lakes and river basins. The immediate margins of saline lakes are bare while open waters support a rich community of phytoplankton, which is dominated by filamentous blue-green algae Spirulina platensis (Vareschi and Jacob 1985). Algal blooms of this species can last for several years, and maintain photosynthetic rates that are close to the highest measured for terrestrial tropical plant communities (Livingstone & Melack 1984).

Freshwater lake edges contain diverse emergent, submerged, and floating macrophytes. Swamp vegetation dominates the lake edges and the mouths of affluent rivers. Tall sedges such as Cyperus papyrus, Phragmites mauritianus and Typha domingensis, and T. capensis, and tall grasses such as Panicum ripens, Echinochloa pyramidalis and E. scabra dominate permanent and seasonal swamps and floodplains (Wetlands International 2002). In the open water, characteristic submerged plants are Nymphaea spp. and floating plants are Pistia stratiotes, Salvinia molesta, and Eichornia crassipes.

Terrestrial habitats

Soda flats with highly alkaline soils are often found near bare mudflats. These soda flats support tall grassland communities, which are characterised by Sporobolus spicatus, short grassland community dominated by Diplachne fusca and sedge formations dominated by Cyperus laevigatus in permanently wet spots. Next to the grassland community lies a belt of Acacia-Commiphora woodland, which may contain Acacia xanthophloea, Acacia tortilis, Tamarix nilotica, and Combretum spp. (Sarunday 1999). Effluent rivers have gallery woodland and flood plains dominated by grasses and tall trees of Acacia tortilis, Acacia seyal and Acacia xanthophloea especially along the river valleys and in areas where underground water covers close to the surface on the floodplain.

The terrestrial areas immediately surrounding the freshwater lakes are covered by a mosaic of bushland, acacia woodland and grassland. As the elevation increases, dense acacia forest occurs, containing such species as Acacia albida, Acacia polycanta and Albizia versicolor (Mwalyosi 1991). The streams feeding the fresh water lakes have sparse gallery forest with tree species such as Tamarindus indica, Ficus sycomorus, and Phoenix reclinata (Loth & Prins 1986; Mutanga et al. 2000).

Description of endemic fishes

There is a small species flock from Lake Natron, composed of Oreochromis alcalicus, O. ndalalani, and O. latilabris. The endemic fish species are capable of surviving in waters of high temperatures and salinity. The three species endemic to Lake Natron and O. grahami, endemic to Lake Magadi, have all been documented to live in waters up to 40o C.  Lakes Kitangiri, Eyasi, Singida and Manyara host the endemic Oreochromis amphimelas in hot springs along their lake margins (FishBase 2001).

Ecological phenomena

This ecoregion is globally recognized for its diversity and density of wetland birds. Wetland bird congregations top one million at four sites (Lake Nakuru, Lake Bogoria, Lake Manyara, and Lake Natron and Engaruku basin) within the ecoregion (Fishpool & Evans 2001). Over two million birds have been documented at Lake Manyara alone (Fishpool & Evans 2001).

Justification for delineation

The formation of the East African Rift led to the isolation of many of the ecoregion’s lakes from other lakes and rivers. The rift formed when tectonic plates below Somalia and the rest of Africa began separating (Lévêque 1997).  The plates are still separating, and the walls of the Rift Valley drift apart at a rate of four mm per year (Cromie 1982). Historic connections existed between some lakes; for example, about 6,000 to 13,000 years ago Lakes Naivasha, Elementaita, and Nakuru were part of a larger lake that may have later contracted due to changes in climate (Hughes & Hughes 1992; International Lake Environment Committee 2001). Thus, this ecoregion is delineated to include the small lakes within the Gregory Rift Valley. The ecoregion is distinguished by its large congregations of lesser flaminogos and the endemic fauna that inhabit its saline and freshwater lakes.

Level of taxonomic exploration

Good


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