The Sudd
Here is a proposal for a slightly modified flight plan from Malakal to Juba in the South Sudan.
It first goes west along a bend in the White Nile to Lake No, the center of the Nuer people. Then it heads south-southeast along the Sudd (in Arabic, "the barrier"). This is a huge swamp covering one of the largest wetlands in the world. The waterways are covered with solid vegetation mats that made it almost impossible to navigate the Nile south from Lake No and Malakal. Historically, this barrier prevented the powers of the lower Nile from penetrating further southward. In particular, the British found it difficult to connect Egypt and Central Africa.
Between Mongalla (near Juba) and Malakal, the flow of Nile drops in half. Wikipedia: the "Sudd area, consisting of various meandering channels, lagoons, reed and papyrus fields, loses half of the inflowing water through evapotranspiration in the permanent and seasonal floodplains." The water, and the nearby grasslands, provide a useful ecology for the pastoral Dinka and Nuer populations who migrate to herd cattle in the dry and raise grain in the wet seasons. Here lie the roots of the South Sudan's culture.