The northern part of the Senegal coast has dunes from Cap Vert to Saint-Louis, but to the south are muddy estuaries. Behind the coast is a sandy plain, which extends north to the floodplain of the Senegal River. The Casamance region in the south, isolated from the rest of Senegal by the Republic of the Gambia, is low but more varied in relief, while to the southeast lie the Tamgué foothills, which rise to a maximum altitude of 581 m (1,906 ft). Much of the northwest of Senegal (known as the Ferlo) is semidesert, but the center and most of the south, except for the forest of Casamance, are open savanna country. The major rivers—the Senegal, Saloum, Gambie, and Casamance—flow from east to west.