Stream
Onilahy River
400 km from source to mouth · Madagascar.
MadagascarFlows into Mozambique Channel3rd longest in Madagascar
In numbers
Length
400km
Average flow
180m³/s
Basin
32,000km²
Drop
876m
from 876 m to 0 m
Country
1
The journey
From source to mouth
400 km of river and 2 stops on the way: 2 towns and cities on its banks, dams, bridges and the tributaries that feed it.
- 0 kmSourceat 876 m
- 73 kmBetroka18,350 people
- 346 kmAntanimena4,000 people
- 400 kmMozambique Channelat 0 m
Water level along the river (RiverATLAS; the height is exaggerated). Towns: GeoNames and Wikidata; bridges, dams and tributaries: Wikidata.
River in Madagascar
At the mouth it carries 181 m³/s on average: an Olympic pool every 14 seconds. It drops from 876 m to 0 m, 2.2 m per kilometre on average. Its basin covers 32,000 km², about the size of BE. 487,000 people live in it. Highest water: from January to February; lowest: from June to September. No large dam holds it back: it is one of the world's free-flowing rivers.
Where to start
- BetrokaLargest city on its banks · 18,350 people
- Tôlanaro Airport (FTU)Nearest airport with scheduled flights, 216 km from the cityCar hire at TNR airport (Antananarivo)Discover Cars · Affiliate link↗
- On the water2 ferry piers, marinas, boat rentals and boat ramps along the river (OpenStreetMap)
- From April to AugustThe best months to visit
Onilahy River, month by month
Highest water: from January to February
Lowest water: from June to September. Best months to visit: from April to August.
From about 36 m³/s in the lowest month to 470 m³/s in the highest. An estimate: the monthly flow is modelled from the lowest and highest months of WaterGAP (RiverATLAS, 1971–2000) and the rain and snow along the river (WorldClim). Dams change the real rhythm. The chip is the average air temperature; below it, the month's rain.
Plan the trip
Plan the trip: boat trips, a bed by the river, a car
Some links above are affiliate links: if you book through them we may earn a commission, at no extra cost to you. It never changes what the data on this page says.
The basin
Onilahy River: the basin in numbers
32,381 km² of land send their water to this river. This is what lives on it.
- 487,000people live in the basin
- 15/km²people per km²
- 654maverage altitude of the basin
- 754mmof rain a year (basin average)
- 22.7°Caverage air temperature
- 1%protected areas
What covers the basin
- 1% Forest
- 4% Cropland
- 59% Pasture
RiverATLAS (HydroATLAS v1.0, CC BY 4.0): land cover, people, protected areas, glaciers and dams over the whole basin upstream of the mouth.
Along the river
Onilahy River: what you find on the way
- 2Weirs
- 2Ferry piers
From OpenStreetMap, within 2.1 km of the river. Not mapped is not the same as not there.
Cross-checked with our atlases
Onilahy River in our other atlases
What our sister atlases know about the waterfalls on this river, the lakes it flows through, the beaches at its mouth, the mountains where it rises and the islands off its mouth.
1island off the mouthIslandyas
Islands off the mouth (1)
Within 30 km of the mouth, by Islandyas
- Nosy Ve20 km away
Islandyas is coming soonOpen Islandyas ↗
From space
The source and the mouth, seen from orbit
Sentinel-2 cloudless 2016 by EOX IT Services GmbH (contains modified Copernicus Sentinel data 2016), CC BY 4.0. The glowing line is the river as drawn by RiverATLAS.
Nearby
Rivers nearby
Where these numbers come from
| Name, length, places | Wikidata (Q2465863) · CC0 |
|---|---|
| Course, profile, basin | RiverATLAS v1.0 (HydroATLAS, Linke et al. 2019) · CC BY 4.0 — followed from the source to the mouth given by Wikidata |
| Flow | WaterGAP (RiverATLAS), 1971–2000 · CC BY 4.0 |
| Climate along the river | WorldClim (RiverATLAS), 1970–2000 · CC BY 4.0 |
| Towns, bridges, dams | Wikidata · CC0; GeoNames · CC BY 4.0 |
| Along the river | © OpenStreetMap contributors · ODbL |
| Images | EOX Sentinel-2 cloudless 2016 · CC BY 4.0; Wikimedia Commons |
| Our other atlases | Islandyas — Wikidata · CC0; © OpenStreetMap contributors · ODbL; GeoNames · CC BY 4.0; HydroLAKES · CC BY 4.0; NOAA OISST |
Every source and licence is on the data sources page.
The family
Do you prefer beaches, lakes, mountains, waterfalls, islands, rivers or canyons?
Same idea, other landscapes: field atlases built on open data, month by month, in seven languages.
beaches
Find your beach: sea temperature, sun and swimming month by month.Visit 
lakes
Every lake on Earth: water temperature, ice, depth and the shore.Coming soonVisit 
mountains
Every summit in 3D: summit weather, the 0 °C line, huts and trails.Visit 
waterfalls
Every waterfall: height, flow month by month and when it roars.Coming soonVisit islands
Every island from the sea up: size, sea temperature, ferries and 3D.Coming soonVisit 
rivers
Every river from source to sea: the journey, the flow and the basin in numbers.You are here
canyons
Every canyon cut to scale: depth to the floor, rim and floor temperatures, trails and viewpoints.Coming soonVisit