Bani River

The Bani River is the principal tributary of the Niger River in Mali. The river is formed from the confluence of the Baoulé and Bagoé rivers some 160 km (99 mi) east of Bamako and it merges with the Niger near Mopti. Its length is about 1,100 km (680 mi).

The Bani River has three main tributaries: the Baoulé that rises near Odienné in Côte d'Ivoire and passes just south of Bougouni, the Bagoé River that rises near Boundiali in Côte d'Ivoire and the Banifing-Lotio that drains the region around Sikasso. The drainage basin upstream of Douna has an area of 102,000 km² (39,000 sq mi), 85% of which lies in southern Mali and 15% in northern Côte d'Ivoire.

The annual rainfall varies across the catchment basin with the southern area in Côte d'Ivoire receiving 1,500 mm (59 in) a year while the northern area around Douna receiving only 700 mm (28 in). For the period 1965-1995 the average annual rainfall for the basin was 1,100 mm (43 in). The rainfall is seasonal with most of the rain falling between May and October. The maximum rainfall occurs in August.

The discharge of the Bani River is also highly seasonal, with the maximum flow occurring at the end of September and very little flow between February and June. The river enters the Inland Niger Delta north of San and after the annual rains the river floods (the French word crue is sometimes used) and covers the floodplain.

There is a significant inter-annual variation in the rainfall and, as a consequence, in the quantity of water flowing in the river. The drought that started at the beginning of the 1970s lead to a very large reduction in the flow and up to the present time the volumes are still much lower than those observed in the 1950s and 1960s. The reduction in the flow of the river was much greater than the reduction in the rainfall. The average rainfall for 1981-1989 was 20% less than for 1961-1970 while the discharge of the river was reduced by 75%. The effect of the reduced rainfall was less extreme for other tributaries of the Niger. Over the same period the catchment basin of the Upper Niger suffered a similar 20% reduction in rainfall but the readings at the Koulikoro gauge station were only reduced to 50% of the earlier values. Because of the accumulated groundwater deficit, even in a year with high rainfall, the discharge of the Bani River is less than in the wet decades of the 1950s and 1960s.

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