Onilahy River

Onilahy River
Onilahy River estuary
Map of the Onilahy River watershed
Location
Country	Madagascar
region	Atsimo-Andrefana, Anosy
Cities	Mandrofify, Ambolofoty
Physical characteristics
Source
• location	near Betroka
Mouth	Bay of Saint-Augustin
• location	Saint Augustin, Mozambique Channel
• elevation	0m
Length	525 km (326 mi)
Basin size	31,600 km^2 (12,200 sq mi)[1]
Discharge
• location	Near mouth
• average	(Period: 1971–2000)183 m^3/s (6,500 cu ft/s)[2]
Basin features
Protection status
Ramsar Wetland
Official name	Zones humides de l'Onilahy
Designated	22 May 2017
Reference no.	2304[3]

Onilahy is a river in Atsimo-Andrefana and Anosy (Toliara Province), southern Madagascar. It flows down from the hills near Betroka to the Mozambique Channel. It empties at St. Augustin (23.5666667°S 43.75°E / -23.5666667; 43.75), and into the Bay of Saint-Augustin.

Two species of cichlids are endemic to the river basin, but Ptychochromis onilahy is probably already extinct^[4] and the remaining range of Ptychochromoides betsileanus covers less than 10 square kilometres (3.9 sq mi).^[5][6]

Onilahy Bassin

Geography

Sources of the Onilahy river are near Beadabo. It flows through Ankilimary, to Benenitra, Ehara, Bezaha and Antanimena. It is crossed by the RN 10 near Tameantsoa. The mouth of the Onilahy river is situated in the Indian Ocean at Saint Augustin, Madagascar, 35 km south of Toliara (Tuléar).

Its main affluentes from its south are Sakamena river, Evasy River, Ianapera River, Isoanala river and the Ihazofotsy River. From the north these are Sakondry, Taheza, Sakamare and the Imatoto rivers.

References

1. Onilahy
2. "Madagascar".
3. "Zones humides de l'Onilahy". Ramsar Sites Information Service. Retrieved 25 April 2018.
4. Stiassny, M., and Sparks, J. S. (2006). Phylogeny and Taxonomic Revision of the Endemic Malagasy Genus Ptychochromis (Teleostei: Cichlidae), with the Description of Five New Species and a Diagnosis for Katria, New Genus. American Museum Novitates 3535.
5. Ravelomanana, T.; Sparks, J.S. (2020). "Ptychochromoides betsileanus". IUCN Red List of Threatened Species. 2020: e.T18832A177066640. doi:10.2305/IUCN.UK.2020-3.RLTS.T18832A177066640.en. Retrieved 12 November 2021.
6. Walaszczyk, I.; Marcinowski, R.; Praszkier, T.; Dembicz, K.; Bieńkowska, M. (August 2004). "Biogeographical and stratigraphical significance of the latest Turonian and Early Coniacian inoceramid/ammonite succession of the Manasoa section on the Onilahy River, south-west Madagascar". Cretaceous Research. 25 (4): 543–576. doi:10.1016/j.cretres.2004.05.001. ISSN 0195-6671.