Lake Chew Bahir is at the end of the Ethiopian section of the Great Rift Valley. It lies across the border of South Omo Zone of Southern Nation, Nationalities and Peoples’ Region to the west, and the Borena Zone of Oromiya Region to the east. The southern tip of the lake crosses the border into Kenya. The nearest settlement to the lake, Arbore, is in South Omo Zone, over 130 km south-east of the zonal capital Jinka. The Lake lies in a basin primarily composed of the flood-plains of the Gelana Dulei river, itself formed by the confluence of the Segen and Weyto rivers. Chew Bahir basin and the Gelana Dulei and Weyto valleys are flanked on the west by steep, finely dissected, scarps rising to 1,600 m, the result of large-scale faulting. Lake Chew Bahir is subject to substantial changes in area as a result of variations in river discharge. It often dries out, but the lowest part in the north-east is always moist. As there is no outlet, all water entering the lake is lost by evaporation. Over the past century, Chew Bahir has varied from swamp to shallow open water with a maximum depth of 7.5 m and a surface area of up to 2,000 km². The water of Chew Bahir is so highly saline that it cannot be used for either irrigation or domestic purposes. However, to the south-east across the mudflats lie a series of springs: some occur around the base of rock outcrops, but others are isolated on the flats where they support a dense growth of coarse, salt-tolerant sedge. The site supports a range of habitats, including marsh, open water, mudflats, springs, Acacia–Euphorbia bushland, mixed broadleaved scrub with Terminalia spp., scattered Acacia and Acacia scrub. There is a sparse vegetation of salt-tolerant plants throughout the basin. Particularly common is the tall coarse grass, Sporobolus consimilis and the mat-forming Sporobolus spicatus. There are a number of Cyperus species which are also highly salt-tolerant. However, where fresh water enters there are rich swampy areas and pools which are said to have tall Echinochloa spp., other Cyperus spp. and Nymphaea spp. The marshes support a high population of amphibians and snails. The Weyto valley has a very open, dry Acacia savanna, the main trees being A. senegal and A. polyacantha, with A. mellifera forming dense stands along with spiny capparid species and Cadaba rotundifolia. Ground-cover is sparse, comprising a few perennial grasses, some interesting succulents such as Caralluma spp., many opportunistic annuals and some geophytes such as Pancratium tenuifolium which only appears after heavy rain. The Arbore, Tsemay and Hamer peoples inhabit the Weyto valley and Chew Bahir basin. They are basically pastoralists, but grow some crops opportunistically. Access to the freshwater springs in and beside the lake is disputed.
See Box and Table 3 for key species. Estimates of Phoenicopterus minor vary between 4,000 in 1975, to ‘hundreds of thousands’ in 1969. A minimum of 300 (but probably nearer 1,000) Anastomus lamelligerus were noted in a small area of marsh. Substantial numbers of Palearctic waders, and presumably ducks, occur seasonally. Other abundant waterbirds include Sterna nilotica, and various Afrotropical ducks including Dendrocygna bicolor and D. viduata. Significant populations of Porphyrio alleni (more than 20 in one small area) and Charadrius pecuarius occur. The Somali–Masai biome species include Mirafra poecilosterna, Turdoides aylmeri, Batis perkeo, Nectarinia hunteri, Plocepasser donaldsoni, Lamprotornis shelleyi, Speculipastor bicolor, Merops revoilii, Passer gongonensis and Lonchura griseicapilla.
Non-bird biodiversity: The plains around the lake support populations of many mammal species including Equus grevyi (EN). The Chew Bahir basin is an important type-locality for a number of species endemic to the arid and semi-arid conditions of the Ethiopian–Kenyan border area.
Pressure/threats to key biodiversity
The lake is situated within the Chew Bahir Wildlife Reserve, which covers an area of c.273,000 ha, although (as far as is known) no conservation activities have ever been implemented in the reserve. It is even unclear whether the reserve has been formally designated as such. The Weyto valley and the lower Omo have been important for big-game hunting.