What emotions is Langston Hughes trying to evoke in "The Negro Speaks of Rivers"?

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.