Here's why:
* No set rhyme scheme: The poem doesn't follow a specific rhyme pattern.
* No fixed meter: The lines are of varying lengths and don't adhere to a regular metrical pattern.
* Emphasis on rhythm and sound: While it lacks a set structure, Hughes skillfully uses repetition, alliteration, and other sound devices to create a powerful rhythm and flow.
This free verse form allows Hughes to express the speaker's deep connection to history and the vastness of the river's journey in a natural, flowing way.