Difficult to access because of sandbanks, the coast has no natural harbors, river mouths, or islands. Behind the coastline is a network of lagoons, from that of Grand Popo on the Togo border (navigable at all seasons) and joined to Lake Ahémé, to that of Porto-Novo on the east, into which flows Benin's longest river, the Ouémé, navigable for some 200 km (125 mi) of its total of 459 km (285 mi). Besides the Ouémé, the only other major river in the south is the Kouffo, which flows into Lake Ahémé. The Mono, serving from Parahoué to Grand Popo as the boundary with Togo, is navigable for 100 km (62 mi) but subject to torrential floods in the rainy season. Benin's northern rivers, the Mékrou, Alibori, and Sota, which are tributaries of the Niger, and the Pandjari, a tributary of the Volta, are torrential and broken by rocks.
North of the narrow belt of coastal sand is a region of lateritic clay, the main oil palm area, intersected by a marshy depression between Allada and Abomey that stretches east to the Nigerian frontier. North of the hills of Dassa, the height ranges from 60 to 150 m (200–500 ft), broken only by the Atakora Mountains ( Chaine de L'Atakoria ), stretching in a southwesterly direction into Togo.