The heart of the old colonial city is located on a narrow island (just over 2 km long and about 400 m wide) in the Senegal River, 25 km from its mouth. At this point the river is separated from the Atlantic Ocean to the west by a narrow sand spit, the
Langue de Barbarie (300 m wide), which has also been urbanized (the seaside neighborhoods of Ndar Tout and Guet Ndar). Yet a third part of the city, Sor, lies on the eastern mainland and is nearly surrounded by
tidal marshes. Saint-Louis is situated on the
Mauritanian border, though the border crossing is at
Rosso, 100 km upstream.
Three characteristics give Saint-Louis its distinctive geographic appearance: the
Sahel, the marshes and the Langue de Barbarie.
Part of the Sahel, a transitional desertic band that separates “[…] the dunes of the Sahara from the baobabs of the savanna”,
 Saint-Louis’ landscape is characterized by occasional
acacias and is disturbed by sand storms during the dry season.
The marshes are flood basins that form during the rainy season when the river overflows into the countryside, creating ponds and stretches of mangroves that attract birds like
Langue de Barbarie
Langue de Barbarie ("Tongue of Barbary"), a 600 km long stretch of sand from
Nouadhibou in Mauritania to Saint-Louis, over a stretch of 25 km separates the lower
Senegal River from the Atlantic Ocean. Its vegetation mainly consists of
Filao trees, propagated to prevent soil erosion in sandy and salty soils.