That description sounds very much like Murchison Falls, a gorge between Lake Victoria (the source of the White Nile) and Lake Albert in today's Uganda. There's certainly nothing like it in the Sudd.

February 12, 2025 (NA’IMA) – Dozens of civilians have been killed in an attack by the Rapid Support Forces (RSF) on villages in Sudan’s White Nile state, south of the capital Khartoum ...