This word is acceptable for play in the UK dictionary that is being used in the following games:

• Lexulous
• Wordscraper
• Wordosaur

adj. Of or pertaining to rivers or their surrounding environments.

Riparian.

"The wind is coursing through the riverain* town and the tall buildings cast shadows over the tiered and narrow streets, canceling out any warmth that the sun could have offered."

riparian