The Senegal River is West Africa's great Sahel border river: formed where the Bafing and Bakoye, both rising in Guinea, meet at Bafoulabé in Mali, it flows west and north past Kayes and Bakel, then marks the Senegal–Mauritania border for most of its lower course before reaching the Atlantic at Saint-Louis, Senegal. Wikipedia gives its length as 1,086 km, with a drainage basin of 270,000 km², a mean flow of 680 m³/s and an annual discharge of 21.5 km³; sources that measure from the Bafing headwaters give a longer figure. Guinea, Mali, Mauritania and Senegal share it through the Organisation pour la mise en valeur du fleuve Sénégal (OMVS), founded by the last three in 1972 and joined by Guinea in 2005. Its importance comes from timing: almost all its water arrives in a few wet months, and the farmers, herders, fishers and birds of the valley have always lived by that calendar, now reshaped by the Manantali and Diama dams.
June to October: Monsoon Rains on the Fouta Djallon and at Kayes
The river's year begins far to the south. The Fouta Djallon in Guinea, rolling grassland at an average of about 914 m and rising to 1,515 m at Mount Loura, receives a great deal of rain and is the headwater of four major rivers, the Senegal, the Gambia, the Konkouré and the Tinkisso branch of the Niger, which is why it is sometimes called the château d'eau, the water tower, of West Africa. The Bafing and the Falémé start there; the Bakoye rises further east, at about 760 m in the granite Monts Ménien northwest of Siguiri.
Further north the rains are short and concentrated. At Kayes in western Mali, the West African Monsoon brings all of the year's rain between June and October, and August is the wettest month, averaging about 216 mm. Almost nothing falls in the other seven months, and the annual total is only around 650 mm. The town is nicknamed the "pressure cooker of Africa" for its heat, which builds before the rains break. Saint-Louis on the coast has the same two seasons: a rainy season from June to October of heat, humidity and storms, and a dry season from November to May.
Rain falling on the Bafing now lands first in the reservoir of the Manantali Dam, 90 km southeast of Bafoulabé. Work began in 1982 and the dam was finished in 1988, but its power plant was delayed by the 1989–1991 border war and by financing, and it only began supplying electricity to Mali, Senegal and Mauritania in 2001. It produces about 740 GWh a year, well above the 540 GWh expected; around 55% goes to Mali, 30% to Senegal and 15% to Mauritania, and in 2006 it supplied more than 90% of Mali's electricity. The price was high: the dam, its power plant and associated works cost 1.02 billion euros, financed by 16 donors after the World Bank declined in 1979, and the reservoir displaced about 10,000 people. Norway refused to contribute because of concerns about the dam's health effects. Downstream, the river passes the Talari Gorges and the Gouina Falls, where the 140 MW Gouina Hydroelectric Plant, inaugurated in December 2022, runs on Manantali's releases, and then the Félou plant, first built in 1927 and replaced in 2014.
The Flood: Kaédi, the Doué Channel and the Île à Morfil
The rains upstream become the flood downstream. Below Bakel, where the Falémé joins, the river crosses semi-arid country and spreads into a broad floodplain. At Kaédi in Mauritania it receives the Gorgol, and just below the town it splits: the left branch, the Doué, runs parallel to the main stream for about 200 km before the two rejoin a few kilometres below Podor. The long island between them is the Île à Morfil, which appeared on the 1413 chart of Mecia de Viladestes as the "insula de bronch".
Before the dams, this seasonal flood was the economy. The Senegal inundated about 150,000 hectares of its valley in an average year and up to 350,000 hectares in high-flow years. Farmers planted in the moist soil the water left behind, the flood sustained the valley's fisheries, and as it fell back herders brought cattle onto the plain to graze. An estimated 370,000 people lived from flood-recession farming in the valley before Manantali was built.
Manantali replaced the natural flood with an artificial release lasting about two weeks. In 1999 the area flooded in an average year was expected to fall to between 30,000 and 50,000 hectares, and much less in a dry year. The Wikipedia account of the dam reports that flood-recession farming has since disappeared, and only a small share of the affected farmers received irrigated land in compensation. Local fisheries declined sharply at the same time.
Falling Water: Recession Fields, Pastures and the Diawara Clash of 8 April 1989
As the flood drained away each year, the valley became contested ground between the farmers who planted the damp soil and the herders who wanted the same land for pasture. The valley has long been home to Fula, Toucouleur, Wolof, Bambara and Soninké communities and to Arab and Berber Moors, and the river is also the international border. The droughts of the 1980s and the new irrigation schemes opened by the OMVS dams sharpened competition for land that was already scarce.
On 8 April 1989 that competition turned violent at Diawara, in the Bakel department of eastern Senegal. A grazing dispute between Senegalese Soninke farmers and Mauritanian Fulani herders ended with at least two Senegalese killed and 12 to 15 taken hostage; Senegal said Mauritanian border guards fired the shots, while Mauritania said the attackers were farmers. Between 21 and 24 April mobs in Dakar looted Mauritanian-owned shops and killed 61 Mauritanians; on 25 April, remembered as Black Tuesday, Mauritanian soldiers and border guards killed at least 200 Senegalese. About 250,000 people were repatriated across the river in both directions, the border closed on 21 August 1989, and the Mauritania–Senegal Border War lasted until 1991.
November to May: Harmattan, Low Water and the Diama Salt Barrier
From November the rains stop and the Harmattan blows dust off the Sahara. At Kayes average highs climb to nearly 42 °C in April and May, and on 3 April 2024 the town recorded 48.5 °C, the highest temperature ever reported in Mali. The river shrinks, and in the past the falling water let the Atlantic push salt up the channel from Saint-Louis. Low water also explains why the Senegal carries so little traffic. The OMVS studied a 55 m wide navigable channel over the 905 km from Ambidédi in Mali to Saint-Louis, which would give landlocked Mali its own route to the sea, but the goal was judged unfeasible and dropped in 1980, before Manantali was even built. Today most crossings are local ones by pirogue and small ferry, such as the canoe ferry linking Bakel with the village of Gouraye in Mauritania.
The Diama Dam, built between 15 September 1981 and 12 August 1986 about 22 km north of Saint-Louis, now stops that salt wedge, holds fresh water for about 45,000 hectares of irrigation and carries the road between Saint-Louis and Nouakchott; it is the only land bridge between Senegal and Mauritania. Further up, a dam built in 1916 keeps salt out of the Lac de Guiers, a 35 km lake that drains to the river through the canalised Taouey at Richard Toll, supplies fresh water to Dakar by pipeline and irrigates the sugar estates on its north shore. Richard Toll, a French outpost from 1817, grew around a colonial rice scheme begun in 1949 and later the Senegal Sugar Company, and had 73,147 people in 2023.
Dry-season irrigation is what the dams were meant to deliver. Manantali's storage irrigates an extra 78,100 hectares: 54,700 in Senegal, 20,400 in Mauritania and 3,000 in Mali. But most farms take only one crop a year, yields remain low, and Senegal's rice self-sufficiency actually slipped from 23% in 1980 to 20% in 2006. Year-round standing water has also brought disease; bilharzia rose sharply in the irrigated areas.
The dry months are also when Europe's birds arrive. The Djoudj National Bird Sanctuary, established in 1971 on the Senegalese bank of the delta and covering 16,000 hectares, has recorded 395 species and nearly three million birds, and between 5,000 and 10,000 aquatic warblers winter there, the most important wintering site yet found for the species. Since Diama began operating, water levels in the park have fallen and cattails and reeds have spread. Across the river, Mauritania created Diawling National Park in 1991 on 16,000 hectares of former floodplain, where manatees, crocodiles and hippos disappeared after the dam was built.
At the mouth, the river's seasons decide how safe Saint-Louis is. The city sits behind the Langue de Barbarie, a sand spit that separates the river's final stretch from the Atlantic. On 3 October 2003, near the end of the rainy season, a 4-metre channel was cut through the spit to relieve flooding in the city; it widened to 800 m almost at once, cut off the southern tip as an island, and by January 2020 was 6 km wide, swallowing villages and resorts. The national park on the spit became a Ramsar site in 2021, and a 2011 documentary called Saint-Louis the African city most threatened by rising seas.
Every season on the Senegal is ultimately set by the Sahel's rain, which our guide to the Sahel region explains, while the river's two banks are covered in our country guides to Senegal and Mauritania.