Except for the prolongation of the Guinea Highlands (in the northwest, from Man to Odienné), which has peaks of over 1,000 m (3,280 ft), the greater part of Côte d'Ivoire is a vast plateau, tilted gently toward the Atlantic. It is drained by four major rivers running roughly parallel from north to south—the Cavally (on the Liberian frontier), Sassandra, Bandama, and Komoé. They are not of much value for transportation, since they are sluggish in the dry season, broken by numerous falls and rapids, and subject to torrential flooding in the rainy season. Lake Kossou (Lac de Kossou), in the center of the country, has been formed by the impoundment of the Bandama. From Ghana to Fresco, the coast is almost a straight line, flat and sandy, with a series of deep lagoons behind it; from Fresco to the Liberian frontier, it is more broken, with small cliffs and rocky outcrops.