In the sentence:
The swiftly flowing stream was icy cold.
The verb is ‘flowing’ which is a gerund. It is used as the subject of the sentence.Gerund are verbs used as subjects in a sentence with the root word added by ‘-ing’. Verbs are simply known as the ‘action’ words – may it be mental, physical or mechanical. When verbs are paired with auxiliaries (helping verbs), they are known as verb phrase. These helping verbs always go first before the actual verb.