If you were the river, would you be enlightenment or would you know enlightenment? In other words, what’s up with the river? What is it’s relation to enlightenment?

Yup, he sure loves the river. I think the river symbolizes both unity and change, note Siddhartha returns to the river whenever his life changes. Consider when he leaves the Samanas or renounces all his wealth, he always goes to his favourite river. I suppose the river becomes a metaphor for tranquility despite change. Both time and existence are personified by the river.