Final answer:
The literary term for the sentence in question is Personification, as it attributes human qualities to America by comparing it to a river.
Step-by-step explanation:
The literary term that describes the sentence "America was a river, roaring along, unmindful of the past" is Personification. Personification is a figure of speech where non-human things are given human attributes. In this sentence, America is being compared to a river, which is given the human quality of being unmindful. This suggests a dynamic, unstoppable force that is indifferent to what has come before, much like a river that flows regardless of its past. Mark Twain's use of personification is evident in his writings as he often gives human characteristics to nature to convey deeper meanings, as is the case with the river scenes in Huckleberry Finn.