The underlined phrase for this question is: "fog rolling down the river"
ANSWER: Absolute phrase
An absolute phrase acts as a modifier to a sentence. It is not connected by any conjunctions and it also cannot contain any finite verb (verbs in the past or present tense).
We can see that the underlined phrase meets all of the definition of an absolute phrase. Its verb, rolling, ends in -ing and is therefore a nonfinite verb. The phrase also modifies the entire sentence, adding information about the fog.