As a preface to his poem, "The Negro Speaks of Rivers," Langston Hughes wrote,

I had been in to dinner early that afternoon on the train.  Now it was just sunset, and we crossed the Mississippi slowly, over a long bridge.  I looked out the window of the Pullman at the great muddy river flowing down toward the heart of the South, and I began to think what that river, the old Mississippi, had meant to Negroes in the past--how to be sold down the river was the worst fate that could overtake a slave in times of bondage.  Then I remembered how Abraham Lincoln had made a trip down the Mississippi on a raft to New Orleans, and how he had seen slavery at its worst, and had decided within himself that it should be removed from American life.  Then I began to think about other rivers in our past.....

"My soul has grown deep like the rivers," Hughes writes in the last line of his poem. And, this seems to be the controlling metaphor for this poem.  The Negro is from the ancient source of life, symbolized by the Euphrates; he has lived by the Congo and the great, long Nile as well as the mighty Mississippi River.  There is a richness to the black experience expressed in lines 4-7 in which these rivers are mentioned. Hughes use of repetition serves to give this experience continuity, as well.

The black people will survive because their souls have grown deep over the centuries and they will continue to last, just as the rivers have.  Langston Hughes poignantly reminds many of the disenfranchised in the United States of their rich and lasting heritage so that they will remain strong.

I would say that one of the primary emotions that Hughes is trying to express is the idea of longevity over time.  There is an emotion of reflection and rumination that Hughes is trying to bring out in the poem.  As the previous thoughts suggest, this reflects a sense of pride in what it means to be Black.  I would also think that part of this is to reach back and define Black History as something connected to the history of the world.  In the opening line of "I've seen rivers," Hughes is trying to bring out the emotion that allows one to see the world in its expansive vision and form, with a Black voice present all the way through.  I think that in doing so, Hughes seeks to evoke the emotions of pain, melancholy, joy, and elation as part of the Black experience or in what it means to be Black.  In this light, Hughes speaks to something that is different than other thinkers, in defining Black History as something not only contingent in slavery.  Rather, it is a realm where individuals can understand that the history of all nations is the history of Black individuals and their contributions.  There is a sense of expansive connection, an emotion of understanding that fully emerges upon reading the poem.

To me, Hughes is probably trying to evoke a couple different emotions depending on who is reading the poem.

I think that Hughes is trying to evoke pride in African Americans who read this poem,  He does this by his references to the antiquity of the black "race."  By talking about how he (and all blacks, by extension) have known the pyramids, etc, he is trying to get black people to feel proud of their race.

I think that he is doing something different for non-black readers.  I think that he is trying to evoke a feeling of wonder and respect.  The words he chooses evoke a sense of the depths of time.  He talks about the rivers that are older than human blood.  He talks about dawns being young.  This makes us feel a sense of awe at how much time he is talking about.  Then, by connecting this depth of time to the black experience, he is trying to make us feel a greater amount of respect for black people.