Boundaries

Numerous highly productive lakes lie in the Eastern Rift valley that cleaves the eastern and western sections of the high altitude Ethiopian dome and extends from the edge of the xeric, Red Sea Coastal ecoregion [527] in the north to Lake Awassa in the south. 

Freshwater habitats

Five major lakes and several rivers lie within this ecoregion. Lake Zwai, the most northerly lake, is located about 160 km south of Addis Ababa at an altitude of 1,840 m. The lake covers 434 km² and its average depth is 2.5 meters (Balarin 1986). Extensive marshes of Papyrus (Cyperus papyrus) border the lake and produce large numbers of mosquitoes (Anopheles pharoensis, A. mauritianus and Taeniorhynchus uniformis)(Omer-Cooper 1930). The Makki River flows into the lake from the northwest and the River Kattar flows into it from the northeast. The lake’s waters flow out through the River Sucsuci into Lake Abijata.

The next three lakes in the chain are Lake Langano, Lake Abijata, and Lake Shala. Lake Langano, located at an altitude of 1,582 m, covers 241 km², and has a maximum depth of 47.9 m and a mean depth of 17 m. Salinity is 1.88 g per liter (Wood & Talling 1988). Lake Langano receives most of its water from small rivers that drain from the Arsi Mountains, which make up the eastern wall of the Rift valley. The only outlet from the lake is the Hora Kelo River, which flows into Lake Abijata (Ethiopian Wildlife and Natural History Society 1996). Lake Abijata is found at an altitude of about 1,600 m and is an alkaline lake. The shores slope gradually, and are muddy, with areas of Juncus vegetation. The depth is about 10 m, and the bottom of the lake is sandy. Three rivers feed Lake Abijata: Gogessa, Bulbula, and Hora Kelo. Water from Lake Zwai also flows into Lake Abijata through River Sucsuci. Lake Abijata has no outlet, and it loses its water by evaporation only. Lake Shala, located at 7^o28’ N 38^o30’E, is at 266 m the deepest of the Ethiopian Rift lakes. It is 28 km long and 15 km wide, and is surrounded primarily by Pleistocene volcanic rocks. The eastern and western shores are covered by lacustrine deposits and Holocene sands, occasionally blackened by obsidian detritus (Mohr 1961). The great depth of Lake Shala may be related to the origin of the basin by intense faulting. Two permanent creeks and several seasonal streams flow into its closed basin.

Lake Awassa has a smaller surface area than Shala and is completely enclosed by faulting. Located at an altitude of 1,680 m, the lake lies south of the other lakes in the ecoregion. It has a surface area of 88 km², a maximum depth of 22 m, and an average depth of 11 m. Lake Awassa is a polymictic lake. The water is murky and alkaline with a pH of between 8.75 and 9.05 (Tudorancea et al. 1988). Like Lakes Abijata and Shala, it is a terminal lake without any visible outlet. Its main tributary is the Tikur Wuha River, which drains swampy Lake Shallo. An extensive belt of submergent and emergent rooted vegetation, which extends about 150 m offshore, covers the littoral zone.

The flora varies among the different lakes. Three major groups of algae dominate: Chlorophyceae, Cyanophyceae and Diatomophyceae (Tudorancea et al. 1999). The most common emergent plants are Scirpus spp., Typha angustifolia, Paspalidium germinatum, and Phragmites sp. Nymphaea coerulea and Potamogeton spp. are the dominant species of floating and submerged vegetation. Abijata and Shala Lakes lack aquatic macrophytes.

Terrestrial habitats

The vegetation to the east and south of Lake Shala is Acacia-Euphorbia savanna. The most common trees are the woodland acacias Acacia etbaica, A. tortilis, and Euphorbia abyssinica, and bushes of Maytenus senegalensis. There is also a rich grass and herb flora (Ethiopian Wildlife and Natural History Society 1996). Beds of bulrushes and sesbania occur where the hot springs and rivers enter the lake, but most of the shore has steep cliffs.

Description of endemic fishes

Three endemic fish, Barbus ethiopicus, B. microterolepis, and Garra makiensis, inhabit Lake Zwai and its adjacent rivers.

Other noteworthy fishes

Introduced species of Tilapia zilli, Clarias gariepinus, and carps are present in Lake Zwai.

Justification for delineation

This ecoregion is defined by the northern lakes of the Ethiopian rift valley and distinguished by lakes with a distinctive fauna when compared to the more southern rift valley lakes of Chamo and Abaya. The fish fauna in the northern lakes appears to have been derived from Awash and associated rivers, while the fish fauna of the latter is Nilo-Sudanic. These faunal affinities may be explained by the tumultuous geologic past of the ecoregion, which was exposed to six volcanic events between the Oligocene and the present (Woldegabriel et al. 1990). Analyses of invertebrate and fish fossils found in the sediments of the lakes indicate that Lakes Zwai, Abijata, Langano and Shala were once united into a single freshwater lake draining northward into the Awash River (Grove et al. 1975; Gasse & Street 1978). The present-day lakes are the result of subsequent tectonic or volcanic activity (Tudorancea et al. 1999).

Level of taxonomic exploration

Fair. Although considerable baseline information is still lacking, this is one of the most studied ecoregions in Ethiopia. Physical and chemical features of some of the lakes are available in Wood et al. (1978) and Wood and Talling (1988). Zooplankton and phytoplankton studies of Lake Awassa are found in Mengistou and Fernando (1991a; 1991b), Mengistou et al. (1991), and Kifle and Belay (1990). Tudorancea and Harrison (1988), Tudorancea and Zullini (1989) and Tudorancea et al. (1989) studied the benthic communities of some of these lakes. Teferra (1987; Teferra 1988, 1989), Tadesse (1988), Teferra and Fernando (1989), Admassu (1994), and Tudorancea et al. (1988) have studied the biology of Oreochromis niloticus, the dominant fish species in these lakes. Nevertheless, the invertebrate fauna, the phytoplankton, and the macrophytes of many of the lakes have not been identified or studied. Moreover, the biodiversity of the rivers and streams is virtually unknown.