Who is the "I" of the poem?

The "I" of the poem is the "Negro" referenced in the poem's title. He is literally a person who is talking about rivers. He does not seem to be one particular black man but, rather, a sort of composite of all black people who have ever lived. He says that he "bathed in the Euphrates when dawns were young," but he also "heard the singing of the Mississippi" during Abraham Lincoln's lifetime as well. It is not physically possible for a person to have been around at the dawn of time and also alive in the middle of the nineteenth century, when Lincoln was alive. Therefore, the speaker seems to be a rather mystical representative of all black peoples, raising them to mythic proportion with allusions and associations.