sentences:  The Ngounié River Valley is formed between the forest-covered Du Chaillu Hills and the Ikoundou Mountains, and has grassy vegetation.The region within this valley is also known as Ngounié.Its tributaries include Louetsié, which passes through Lebamba and Mbigou; Ikoy, whose main tributary are the Ikobe River and the Oumba River; Dollé, which passes through Ndendé; as well as the Ogoulou, Ngongo and Ovigui rivers.