The underlying concept is: The waterway was sombre. "To flow sombrely" would describe the manner of flowing, but  there is no sombre way of flowing or a happy way of flowing. Water can flow slowly, sluggishly, fast, but not sombrely.

The problem of adjectives after verbs in adverb position is very tricky and it takes some time to get a feeling for this problem. The school rule "after a verb follows an adverb" is really only a school rule. There should be added in brackets: (But there are a lot of cases where an adjective follows because the intent is not to describe the manner.)