I've known rivers:
I've known rivers ancient as the world and older than the
Flow of human blood in human veins.
My soul has grown deep like the rivers.
I bathed in the Euphrates when dawns were young.
I built my hut near the Congo and it lulled me to sleep.
I looked upon the Nile and raised the pyramids above it.
I heard the singing of the Mississippi when Abe Lincoln went down to New Orleans, and I've seen its muddy bosom turn all golden in the sunset.
The Euphrates was an ancient Mesopotamian river. Here it symbolizes the beginning of civilization. And dawn is (I hope you know) the beginning of the day, which here represents the same thing. 
 
 The idea of the “dawn” of time, and the “dawn” of the day connects, as the river and the blood do, the soul with the earth. Just as Euphrates is the dawn of civilization, the Black man rises at dawn to work the same earth he is “made of”. Hughes is constantly drawing parallels between the man and the earth, demonstrating the permanence, resilience and wisdom.
The Negro Speaks of Rivers