I looked upon the Nile and raised the pyramids above it. (7)
We see the strength of our speaker in this moment, imagining him single-handedly building these great monuments. We admire the creativity and innovation he possesses in doing so. Notice how he doesn’t say, "I looked upon the Nile and built the pyramids above it." Instead, he uses the verb "to raise" which sounds much more mystical to us than "to build." However, we also remember that it was once widely believed (thanks to the Egyptian historian, Herodotus) that 100,000 slaves were forced to build pyramids. In this light, we watch our speaker move from living freely by the Congo River to a life of slavery by the Nile.
I heard the singing of the Mississippi when Abe Lincoln went down to New Orleans (8-9)
The Mississippi River was used to help transport slaves, and our speaker refers here to the moment at which a nineteen year-old Abe Lincoln witnessed slavery and the slave trade for the first time. The Mississippi is the last of the four rivers that our speaker names, and it is closely tied to the history of slavery. In this way, we see the trajectory of freedom to enslavement to emancipation in the history of each river.
and I’ve seen its muddy bosom turn all golden in the sunset. (9-10)
Why does our speaker use the term "muddy" here? Why doesn’t he say, "and I saw its muddy/ bosom." It’s as though the transformation from muddy to golden has happened over and over again, and he has witnessed this process over and over again. If we understand this transformation as a metaphor for the emancipation of slaves, we might also understand that such a transformation continues to take place as people continue to be subject to inequality, and as they continue to fight for freedom and equality.