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.
Hughes is connecting the life of the American Negro to that of the continent of Africa. He makes it clear from the beginning that this poem is about pointing out the history of black people in America – Negro – and how this history flows across the seas back to the ancient rivers which witnessed the birth of humanity. 
 The use of the word ‘known’ instead of ‘know’ suggests the speaker’s knowledge is intimate. He didn’t just pass through the rivers as a quiet observer but rather has had a deep relationship (good or bad) with them.
The Negro Speaks of Rivers