The title of "The Negro Speaks of Rivers" lets us know early on that the story that follows is told from the perspective of a member of the black community. Our speaker begins telling the tale of the birthplace of all civilization by taking us to the Euphrates River, but he ends at a time and place tied to the history of slavery and racism in America: when Abe Lincoln rode a boat down the Mississippi River, witnessing for the first time the horrors of slavery. Our speaker watches the "muddy" Mississippi turn "golden" in the light of the setting sun, suggesting the transformation from slavery to freedom that many Americans experienced after the Civil War. He celebrates the black community in this way.
The image of the Mississippi River turning to gold is a symbol for the freeing of slaves.