I’ve known rivers ancient as the world and older than the flow of human blood in human veins. (2-3)
Memory is a human thing, right? Without humans, can there be memory? Our speaker connects himself to something that is before human memory and history. He connects himself to the bedrock and foundation of civilization.
I bathed in the Euphrates when dawns were young. (5)
Not only does our speaker know old rivers, but he grew up with them too. He was around at the birth of civilization in Mesopotamia.
I built my hut near the Congo and it lulled me to sleep. (6)
Our speaker is not a god or a divine creature. He is distinctly human and he does things like bathe, build, and sleep.