"To dig the channels through which the moral life of his countrymen might flow" is an example of figurative language.
Figurative language employs figures of speech in order to be more effective, persuasive, and impactful. Figures of speech, including metaphors, similes, and allusions go beyond the literal meanings of the words to give readers new insights. Figurative language may appear in multiple forms with the use of different literary and rhetorical devices.