Ndiaël basin (including the Trois Marigots)

Year of compilation: 2001

Site description
The site consists of the alluvial Ndiaël basin and the ‘Trois Marigots’ marshes which lie in the flood-plain of the Senegal river, about 60 km north-east of St Louis. The basin lies south of the main road (RN3) linking St Louis and the other northern towns along the Senegal river, about 3 km east of Ross-Bethio and 20 km south-east of the Djoudj wetlands (site SN001). The ‘Trois Marigots’ area lies immediately to the south-west of Ndiaël basin. It consists of three marshes in parallel depressions separated by dunes, varying in length from c.15 km to c.20 km and each only a few hundred metres wide. In the past, and under natural conditions, the basin filled with water annually, as the Senegal river flooded out onto its flood-plain between July and October, and the habitats included areas of Acacia spp. scrub and open water. The basin was fed directly from the river and from nearby Lac de Guiers (site SN003) and, when flooded, the site attracted large numbers of waterbirds; both Afrotropical and Palearctic migrant species.

However, the extensive engineering works (dams, embankments, sluices, etc.) that have been carried out in order to promote irrigated agriculture (mainly rice) in the flood-plain, and described under sites SN001 and SN004, have resulted in the Ndiaël basin remaining largely dry since the 1960s. The water-supply to the Trois Marigots was also greatly reduced by management works at Ndiaoudoum, cutting off this source of flow into the basin. The soils in the basin are impermeable and saline and the vegetation is dominated by annual grasses (Gramineae), such as Paspalum, Panicum and Egragrostis spp., with small Tamarix sp. trees and Typha sp. along the banks of canals and ditches. Some irrigated areas of the site are used for rice cultivation, but the traditional activities of fishing and flood-recession agriculture/pastoralism have declined along with the regime of natural floods. Starting in 1993, a major effort has been made by the Senegalese Direction des Eaux et Forêts and the French ‘Oiseaux Migrateurs du Paléarctique Occidental’ (OMPO) to re-flood the basin annually (see ‘Conservation issues’).

Key biodiversity
See Box for key species. The site appears to have been extremely important in the past for a wide variety of resident and migratory waterbirds. There are counts from the 1960s and 1970s of tens of thousands of Anas querquedula and A. acuta and large numbers of Phoenicopterus ruber (5,000), Philomachus pugnax (200,000) and Larus genei (200). Following the re-flooding of the basin and marshes since 1994, recent counts show that even periodic availability of water makes the site very attractive again for a range of herons, ducks and waders, especially wintering and staging Palearctic migrants. Thus it can be said once again to hold regularly in excess of 20,000 waterbirds. Lower water-levels appear to favour waders, while ducks are more attracted to deeper water conditions. In addition to those species in the Box, there are regular records of other species, including Pelecanus onocrotalus, Casmerodius albus, Plegadis falcinellus, Platalea leucorodia (including a count of 150 birds near the site at Ross-Bethio in 1987) and Phoenicopterus ruber. A maximum count of 10,935 Limosa limosa was recorded on the site in 1993, but it appears that usually no more than 3,000–5,000 birds of this species overwinter in the whole Senegal river delta. Anas acuta has also been recorded recently in numbers (7,860 in 1996) close to the IBA threshold (A4i) for this species. Four of the 12 species of the Sahel biome (A03) that occur in Senegal are recorded from this site (see Table 2).

The site forms part of the wider complex of Senegal river delta wetlands and there is considerable movement of birds between this site and others, including Djoudj wetlands (site SN001), Lac de Guiers (SN003), River Senegal (SN004), Guembeul Avifaunal Reserve and St Louis lagoons (SN005) and sites on the northern side of the river in Mauritania. For example, a ‘pre-roost’ of 134,000 Philomachus pugnax was recorded on the site en route to Djoudj wetlands in 1997, and there are frequent exchanges of wintering Phoenicopterus ruber between all the Senegalese sites and Aftout Es Saheli in Mauritania.

Non-bird biodiversity: None known to BirdLife International.

Pressure/threats to key biodiversity
The ‘Trois Marigots’ area is a Hunting Reserve (‘Zone d’intérêt cynégétique’). Part of the Ndiaël basin is an Avifaunal Reserve (Réserve de faune du Ndiaël, 46,550 ha) and was designated as a Ramsar Site in 1977 and placed on the ‘Montreux Record’ (the list of the world’s most threatened Ramsar Sites) in 1987. At the time of designation, the site had been dry for many years and it was accorded Ramsar status on the basis that the government of Senegal, together with international partners, would implement a hydrological restoration plan. This began in 1993/94 with the construction of a canal linking the northernmost of the Trois Marigots with the south-western corner of the basin. This has resulted in re-flooding of parts of the basin, notably in 1996/97, when there were high flood levels throughout the flood-plain. However, the flow of water into the Trois Marigots and subsequently into the Ndiaël basin is very dependent on flood levels in the main river and the operation of sluices and barrages downstream, which control water-supply to the town of St Louis. Currently, partial inundation of the basin will only happen when flood levels are high. The site is now the subject of a management plan prepared by the Senegalese Direction des Eaux et Forêts and the French Conseil International de la Chasse (CIC) working group ‘Oiseaux Migrateurs du Paléarctique Occidental’ (OMPO). The main objectives are to restore the international importance of the site for birds, to encourage traditional activities such as fishing and livestock-rearing at sustainable levels and to reforest suitable areas for erosion control and fuelwood-supply. The plan (1999 to 2003) also includes a programme of water-level and ecological monitoring to gauge the success of the re-flooding efforts and impact, especially on birds.

Recommended citation
BirdLife International (2022) Important Bird Areas factsheet: Ndiaël basin (including the Trois Marigots). Downloaded from on 30/06/2022.