... Speaking About Rivers...

"The Negro Speaks of Rivers" is a poem inspired by a trip he was on to visit his father in Mexico, just as he passed a river. How in slavery time, slaves were often sold on the rivers. The Mississippi River is mentioned because he remembered reading about Lincoln traveling through there and seeing people being sold and bought, a moment Lincoln would never forget and perhaps grew to become a significant factor in the signing of the Emancipation Proclamation.

As Hughes states: "I've known rivers. I've known rivers ancient as the world and older than the flow of human blood in human veins." These rivers he has come to known is man, specifically "Negroes", all  of them. He continues  to speak n behalf of them, presenting their history through the rivers. The Euphrates and the Tigris (Fertile Crescent), The Congo and the Nile, (rivers in Africa), and the last one is the Mississippi, which was explained earlier. 

The poem repeats "rivers" very often, here and there, setting its jazzy rhythm. Near the end, "I've known rivers / Ancient, dusky rivers", dusky as in dark, night, black. "My soul has grown deep like the rivers", suggesting the history, the suffering and  the journey of his people.