Tana, Athi & Coastal Drainages
Dalmas Oyugi, National Museum of Kenya, Nairobi, Kenya and M.L. Thieme, Conservation Science Program, WWF-US, Washington, DC, USA
Major Habitat Type
Tropical and subtropical coastal rivers
Drainages flowing into
Main rivers to other water bodies
The Tana River is the largest and longest river in Kenya (about 1,000 km) and the Tana River system drains the southern and eastern slopes of Mount Kenya and the eastern slopes of the Aberdare Ranges. Flooding of the river’s lower reaches is due to heavy rainfall on these mountains rather than from local rains in the arid lowlands (Wass 1995; Bennun & Njoroge 1999). The Athi River, called the Galana and Sabaki in its lower course, is the second largest river in Kenya. The upper Athi River drains the eastern slopes of the Aberdare Range; the Tsavo River, its main tributary, drains the Kenyan slopes of Mt. Kilimanjaro.
Draining Mount Kenya and surrounding highlands, the coastal rivers of this ecoregion, with their associated swamps, floodplains, and lakes, host a relatively depauperate fish fauna but a rich avifauna and herpetofauna and include some relict endemic forms of Odonata. This ecoregion encompasses the basins of the Tana and Athi/Galana Rivers and those of several other coastal rivers that flow to the Indian Ocean, including the Goshi, Tiwi, Pemba, Umba, and Ramisi Rivers. Many of the tributaries drain from Mount Kenya, the Aberdare Ranges, and from the northern slopes of Mt. Kilimanjaro. Smaller rivers and streams also drain the coastal margins of Eastern Africa, such as from the Shimba Hills and Arabuko-Sokoke forest. As its name suggests, the ecoregion covers Kenya’s east coast inland to the walls of the Great Rift Valley, and it also extends marginally into northern Tanzania.
The Tana River flows through extensive wetlands and its estuary has one of the best mangrove stands in Kenya (Kairo, Kenya Marine and Fisheries Research Institute, Mombasa, pers. comm.). On its upper course the river flows northwards, then curves south just below the equator. The Tana’s main tributary, the River Sagana, originates from a series of springs among open moorlands on Mount Kenya. As the Tana flows north, it receives the Galole, Hiraman, and Tula tributaries on its left bank. The last cataract on the river occurs at Kora. The river then flows towards Kipini, where it splits into several distributaries and forms a huge delta (Crafter et al. 1992). Oxbow lakes are prevalent along the lower course of the Tana and include Lakes Vukoni, Pongi, Pacha, Lango La Shimba, Bilisa, Idsowe, Harakisa, Moa, Shakababo, Kongolola, Kitumbuini, and Dida Warede. The Tana presently flows into the delta through an artificial channel that was dug for navigation purposes between Belazoni and Ozi about a century ago. Some fresh water flowed through the old Tana River channel until recently, when it was impounded by farmers to provide water for irrigation. The delta is about 50 km long at its base where it opens into Formosa Bay.
From Shakama (80 km upstream from the coast), the Athi River descends about 91 meters and becomes sluggish in flow. It is dotted with seasonal pools that dry out between rainy seasons. There are several permanent lakes high up on the floodplain, including Jilore, Merikano, Mekimba, plus several smaller ones. These lakes are important breeding and nursery grounds for many fish species. The Sabaki River estuary is relatively short and the tidal portion is lined with fragmented mangrove stands and lagoons. As with the Tana, rains and snowmelt from upstream, rather than local rains, are the source of flooding. The Koroni swamp/marsh is located to the northeast of the river mouth.
The Tana River passes through arid and semi-arid zones, meandering through alluvial floodplains and a thin belt of riverine forest. The forests support a diversity of species, including stands of Garcinia livingstonei and Synsepalum brevipes and characteristic species such as Ficus spp., Phoenix reclinata, Acacia robusta, and Blighia unijugata (Burgess & Clarke 2000). The riverine forest that surrounds the Athi River widens towards the sea and joins Arabuko Sokoke forest at Jilore. The bank is lined with macrophytes such as Phragmites, Cyperus,and Saccarum spp., whereas the small lakes are covered with dense Nymphaea, Salvinia, and Pistia.
Description of endemic fishes
About fifteen of the fifty known species of freshwater fish are endemic to this ecoregion. A few of the characteristic species are Protopterus amphibius, P. annectens, Neobola fluviatilis, Synodontis serpentis, S. zanzibaricus, S. manni, Nothobranchius willerti,and Oreochromis spilurus. The endemic Parailia somalensis is known only from the lower 200 miles of the Tana River, and the endemic subspecies Petrocephalus catostoma tanensis is restricted to this river below Garissa (140 m asl) (Whitehead & Greenwood 1959; Whitehead 1962). The catadromous eel Anguilla bengalensis has been recorded far upstream (2700- 3000 m) in the Ragati River in the bamboo zone during the onset of floods (van Someren 1952). There is also a species radiation within the Aplocheilid genus Nothobranchius (Lévêque 1997).
Justification for delineation
This ecoregion encompasses the basins of the Tana and Athi/Galana Rivers and those of several other coastal rivers that flow to the Indian Ocean. A distinctive fish fauna characterizes the basins of the Tana and Athi Rivers, although recent studies show that this fauna may be more similar to that of the Shebelle-Juba catchments than previously thought. For example, the pancake catfish (Pardiglanis tarabinii) (Poll et al. 1972) was recently caught in the Tana River (De Vos 2001). Until March 2000, this species was known only from its holotype, which was from the Juba River in Somalia. Further studies should reveal whether these two ecoregions should be merged. The Athi and Tana Rivers share only two species (Clarotes laticeps and Mormyrus kannume) with the Nile systems. However, these eastern flowing rivers do share affinities with the Lake Victoria fish fauna. There are about 10 fish species (including Petrocephalus catostoma and Chiloglanis sp.) in common, suggesting that the headwaters of the eastern flowing rivers may have originally connected with the Victoria basin before faulting of the Rift Valley (Whitehead & Greenwood 1959; Whitehead 1962). This suggests that the east-west Pliocene hydrographic boundary was initially far to the west of present day Lake Victoria.
Level of taxonomic exploration
Poor. Whitehead (1959) noted that the Tana River is less studied than the Athi River due to inaccessibility. Aside from a recent inventory undertaken in parts of both rivers by the Ichthyology Department of the National Museums of Kenya, there is little current information. The work of Whitehead (1959; 1960; 1962) and Whitehead and Greenwood (1959) is several decades old. Lothar Seegers (Dinslaken, Germany) and Luc De Vos (National Museum of Kenya, Nairobi) are currently completing a checklist of Kenyan freshwater fishes. The Kenya Marine and Fisheries Research Institute (KMFRI) has conducted biotic inventories of the creeks and bays of this ecoregion, with particular attention given to Mida Creek and Gazi Bay.
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