The Southern Ewaso Ng'iro (Brown River) is a river in the

Great Rift Valley The Great Rift Valley () is a series of contiguous geographic depressions, approximately 6,000 or in total length, the definition varying between sources, that runs from the southern Turkish Hatay Province in Asia, through the Red Sea, to Moz ...

Kenya Kenya, officially the Republic of Kenya, is a country located in East Africa. With an estimated population of more than 52.4 million as of mid-2024, Kenya is the 27th-most-populous country in the world and the 7th most populous in Africa. ...

. It plays an important role in the ecology of

Lake Natron Lake Natron is a salt lake, salt or soda lake, alkaline lake located in north Ngorongoro District of Arusha Region in Tanzania with its far northern end crossing into Kenya. It is in the Gregory Rift, which is the eastern branch of the East A ...

, the main regular breeding site for near-threatened

lesser flamingo The lesser flamingo (''Phoeniconaias minor'') is a species of flamingo occurring in sub-Saharan Africa and western India. Birds are occasionally reported from further north, but these are generally considered vagrants. Characteristics The lesse ...

s. Changes to land use in the river's headwaters or in the marshes before the river enters the lake could have a serious impact on this species.

Course

The Ewaso Ng'iro rises on the

Mau Escarpment The Mau Escarpment is a fault scarp running along the western edge of the Great Rift Valley in Kenya. The top of the escarpment reaches approximately 3,000 m (10,000 ft) above sea level, and is over 1,000 m higher than the floor of ...

, where it drains the south part of the

Mau Forest Mau Forest is a forest complex in the East African Rift, Rift Valley of Kenya. It is the largest indigenous montane forest in East Africa. The Mau Forest complex has an area of . The forest area has some of the highest rainfall rates in Kenya. Ma ...

. The forest, which plays an important role in regulating and filtering the inflow to the river, is under threat from logging and land clearance for farming. Destruction would increase sediment loads in the river and cause greater seasonal variance in the volume of water. The river flows south through the rift valley to the east of the

Nguruman Escarpment Nguruman Escarpment is an escarpment in southern Kenya. The escarpment is around 50 kilometers long and elongated in the N-W direction. Its northern edge is approximately 120 km southwest of Nairobi, while the southern edge is near the Tanzani ...

. It crosses the border into

Tanzania Tanzania, officially the United Republic of Tanzania, is a country in East Africa within the African Great Lakes region. It is bordered by Uganda to the northwest; Kenya to the northeast; the Indian Ocean to the east; Mozambique and Malawi to t ...

, where it empties into

. The river, which runs all year round, is the main inflow to the lake. The river once flowed directly into the lake, but in geologically recent times it has been dammed by a horst beside the Shompole volcano. This has caused the waters to spread out into the steadily expanding Engare Ng'iro swamp, where the river deposits its sediment. The sediment-free river water then seeps into the brine lake. The permanent swamp covers about . South of this a seasonal floodplain of about stretches down to Lake Natron and along its eastern shore.

Possible changes

The Lake Natron basin has been designated a Wetlands of International Importance under the

Ramsar Convention The Ramsar Convention on Wetlands of International Importance Especially as Waterfowl Habitat is an international treaty for the conservation and sustainable use of Ramsar site, Ramsar sites (wetlands). It is also known as the Convention on We ...

. However, in the past there have been plans to dam the Ewaso Ng'iro for hydroelectric power generation and for irrigation of the marshlands north of the lake, diverting water from other rivers to increase the flow. The plans would also include creating a variable freshwater lagoon with an area of about . If implemented, the impact on the lake's ecology could be drastic. Reduced salinity and pollution with agri-chemicals could wipe out the blue-green algae that provide food for the

. The lake is the main breeding ground for this near-threatened species. As of 2007 the dam project appeared to be on hold.

References

{{Authority control Rivers of Kenya