Songhai runs the strongest river-and-desert trade economy of the Exploration Age — a civilization that turns Navigable Rivers into a genuine Resource and Gold engine, with trade ships that can't even be plundered while sailing them.
"Tarikh al-Sudan" grants increased Resource Capacity in Cities on Navigable Rivers plus increased Production in those Cities for every Resource assigned to them, growing further in later Ages. After completing the right Civic, Cities can generate Homelands Treasure Convoys on Navigable Rivers, and Songhai gets increased Production toward its associated Wonder.
The Tajiro is a unique Merchant that grants a large Gold bonus when creating a Trade Route that includes at least one Navigable River tile. The Gold Bangles Infantry is a unique Infantry unit with increased Combat Strength on Resource tiles that also gains extra yields when Plundering enemy Trade Routes.
The Caravanserai is a unique Improvement granting Gold plus adjacency bonuses for Rivers and Resources; it must be built on Desert or Plains terrain. The associated Wonder, the Tomb of Askia, adds Gold, increases Resource Capacity, and grants Gold and Production per assigned Resource; it must be built on Desert.