Boundaries

The Inner Niger Delta is located in central Mali in the semi-arid Sahelian zone, just south of the Sahara Desert, roughly situated between Djenné in the south and Tombouctou in the north. Boundaries of the ecoregion follow the general extent of the Delta floodplains and associated grasslands. Dune ridges on the Sahara’s edge funnel the waters of the Inner Delta north and east through Mali.

Topography

The Inner Niger Delta lies in a depression that formed the bed of a large lake during the Quaternary period (Welcomme 1986b). The delta extends for 425 km with an average width of 80 km, tapering into a braided river near Tombouctou where the Niger curves to the east. The floodplain drops only 8 m over its course (Hughes & Hughes 1992) and its topography is a complex mix of submerged lower areas and higher, unflooded areas known as tougérés. A vast network of river channels (mayo’s) with levées separated by low, clay-based floodplains forms networks across the delta. As waters flow through the delta, they pass over Pleistocene and recent alluvium overlying Paleozoic sandstone (Hughes & Hughes 1992). The upper margin of the delta is delimited by the 280 m contour and it is surrounded by sandstone massifs. To the north, huge dunes of the Erg Ouagadou block a former westward course of the river.

Freshwater habitats

Vegetation defines the different habitat types of the delta. Three main plant associations have been identified: submerged and floating plants in shallow or stagnant water, partially submerged and marginal vegetation dominated by grasses, and plants that grow on seasonally exposed sandy soils (Hiernaux 1982; John et al. 1993). Along the rivers, a typical scrub of Mimosa pigra and Salix chevalieri is found, often together with a vegetation of Vetiveria nigritana. Partly floating, long-stemmed grasses – Echinochloa stagnina, E. pyramidalis, Oryza barthii and Andropogon gayanus – dominate in the floodplains. Permanent pools are richer and host the submerged macrophytes, Ceratophyllum spp. and Utricularia spp., in addition to floating Nymphaea spp. The lakes, in particular Lake Debo and Walado Debo in the central part of the delta, are surrounded by Echinochloa spp. and Vossia cuspidata (Dumont 1987). Flooded forests of Acacia kirkii are also characteristic, but increasingly rare due to overharvesting. Though these forests are dominated by A. kirkii, Ziziphus mauritiana may also occur. Algal blooms are common on the lakes and can reduce the water transparency.

Grasses such as Acroceras amplectens, Echinochloa pyramidalis, E. stagnina and Eragrostis atroviriens dominate the low-lying floodplain in the southern half of the delta. Large areas of the flooded delta are occupied by wild rice (Oryza longistaminata) and a characteristic vegetation of E. stagnina, known as bourgou-fields. Bourgou is used as feed for domestic animals and, thus, is often planted by local human populations. Other typical species of the flooded pastures that occur higher in the inundation zone are Vetivera nigritiana and Vossia cuspidata. Along the heavily grazed outer fringes, Andropogon gayanus, Cynodon dactylon and Hyparrhenia dissoluta dominate.

Each year during the rainy season, floodwaters of the Niger and Bani Rivers spill over their banks and the Inner Niger Delta in Mali is inundated to an area of 30,000 km², on average. In contrast, the delta contracts to 3,900 km² or less during the dry season (Welcomme 1986b; Zwarts & Diallo 2002). However, the surface of the inundated zone is highly variable according to river discharge: from 1956-2002 the maximum flooded area varied between 9,500 km² (1984 – severe drought) and 44,000 km² (1957 – high floods) (Quensière 1994).

Rainfall occurs in the Niger’s headwaters from May through September, with a clear peak in August, creating a surge that reaches the inland delta in October (Zwarts & Diallo 2002). As a result of lateral expansion, the flood slows in the delta. It takes one month for the flood to reach Mopti 160 km downstream, and nearly another month to reach Lake Debo where the maximum flood occurs in November-December (Dumont 1987; Laë 1997; Zwarts & Diallo 2002). The extensive swamps and vegetation of the delta filter silt and salt from the water so that the water leaving the delta is clear, low in dissolved salts and silt-free (John et al. 1993). Dry, landlocked Mali is completely dependent on these rivers for its water resources.

Terrestrial habitats

Forests are scarce in the delta, having been heavily exploited, overgrazed, and harvested for fire wood. Trees such as Acacia seyal, Diospyros sp. and Kigelia africana grow on higher levées. The northern half of the delta - north of the Debo-complex is characterized by emergent sand ridges; palms like Hyphaene thebaica, and Borassus aethiopum (Gallais 1967).

Description of endemic fishes

Two species are restricted to the delta and the Upper Niger: Synodontis gobroni and the rapids dwelling Gobiocichla wonderi (Welcomme 1986a). 

Ecological phenomena

The ecoregion is recognized for its continentally important congregations of wetland birds (Fishpool & Evans 2001; Thieme et al. 2005). Maximum densities of wetland birds documented at the Lac Debo-Lac Oualando Debo site exceed one million birds (Fishpool & Evans 2001).

Justification for delineation

The Inner Niger Delta ecoregion is distinguished due to the important habitat it provides for wetland birds, including both Palearctic migrants and Afrotropical residents, and other aquatic fauna (i.e., fish, mammals, amphibians, etc.) that also depend on the extensive floodplains.

Level of taxonomic exploration

Good