The most important metaphor is that of the entire poem. The river's path is being compared to the path of a person travelling through life and trying to stay on the right course even as all the wonders around him/her distract.
In stanza two this is a comparison to the distractions in youth and the want for a beautiful woman as well as for a mother still. The river is being seduced - "The laving laurel turned my tide."
The trees and their shade are being compared to the safety and lessons of a father who shelters us for awhile while we learn and then sends us on our way.
The luminous stones are leading the river to spiritual contemplation; the enlightened path, and the friendly brawl is like a temptation when we are on the right track, a rebirth of our youth for a moment.
The last stanza is about the journey to the ocean which is the duty of the river whether he likes it or not which is much like our earthly obligations like helping others and working.
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