The watershed between the major Congo and Nile drainage basins runs from north to south through Rwanda, with around 80% of the country's area draining into the Nile and 20% into the Congo via the Rusizi River and Lake Tanganyika. The country's longest river is the Nyabarongo, which rises in the south-west, flows north, east, and southeast before merging with the Ruvubu to form the Kagera; the Kagera then flows due north along the eastern border with Tanzania. The Nyabarongo-Kagera eventually drains into Lake Victoria, and its source in Nyungwe Forest is a contender for the as-yet undetermined overall source of the Nile. Rwanda has many lakes, the largest being Lake Kivu. This lake occupies the floor of the Albertine Rift along most of the length of Rwanda's western border, and with a maximum depth of , it is one of the twenty deepest lakes in the world. Other sizeable lakes include Burera, Ruhondo, Muhazi, Rweru, and Ihema, the last being the largest of a string of lakes in the eastern plains of Akagera National Park.