It rises in Guinea from the Bafing River, which is joined in eastern Mali by the Bakoye River. As it enters Senegal, the Falémé River joins it from the south. At high flood stage, water from the Senegal River spreads through a system of channels, sloughs, and adjacent lowlands until most of the valley is a sheet of water, from which the tops of trees appear as green patches and villages stand out as isolated islands.
At the onset of the long dry season, ocean tides extend nearly 483 kilometers (300 miles) upstream. During the rainy season, however, the salty water is forced seaward and the system is refilled with fresh water.
The Gambia River, which rises in Guinea, receives the flow of a perennial river, the Koulountou, which also runs north from Guinea to join it near the Gambian border. Between The Gambia and Guinea-Bissau, the Casamance River drains a narrow basin less than 32 kilometers (20 miles) wide, becoming a broad estuary 104 kilometers (65 miles) from the sea, 10 kilometers (6 miles) wide at the mouth.
The Saloum River and its major tributary, the Siné River, feed into an extensive tidal swamp just north of The Gambia. Only the lower reaches carry water all year, and these are brackish, as the tides penetrate far up the various channels through the swamp.
The largest lake in Senegal is the artificially controlled Lac de Guiers (Guiers Lake). This shallow lake is fed by the Senegal River and extends for an average length of about 80 kilometers (50 miles), averaging about 12 kilometers (8 miles) in width. A dam, as well as a gate on what is known as the Taoué channel, control water flow into this lake. At the highest level, the lake waters reach another 64 to 80 kilometers (40 to 50 miles) southeastward into the Ferlo Valley.
To the north of the Cap Vert peninsula lies Lac Rose (Pink Lake), a shallow saltwater lake occupying a depression behind the coastal dunes. Organisms that live in the lake give it a pinkish color, and villagers extract its salt for commercial purposes.