The coastline consists mostly of a low sandy shore behind which stretches the coastal plain, except in the west, where the forest comes down to the sea. The forest belt, which extends northward from the western coast about 320 km (200 mi) and eastward for a maximum of about 270 km (170 mi), is broken up into heavily wooded hills and steep ridges. North of the forest is undulating savanna drained by the Black Volta and White Volta rivers, which join and flow south to the sea through a narrow gap in the hills. Ghana's highest point is Mount Afadjato at 880 m (2,887 ft) in a range of hills on the eastern border. Apart from the Volta, only the Pra and the Ankobra rivers permanently pierce the sand dunes, most of the other rivers terminating in brackish lagoons. There are no natural harbors. Lake Volta, formed by the impoundment of the Volta behind Akosombo Dam, is the world's largest manmade lake (8,485 sq km/3,276 sq mi).