High Quality Content by WIKIPEDIA articles! Fanno Creek is a 15-mile (24 km) tributary of the Tualatin River in the U.S. state of Oregon. Part of the drainage basin of the Columbia River, its watershed covers about 32 square miles (83 km2) in Multnomah, Washington, and Clackamas counties, including about 7 square miles (18 km2) within the Portland city limits.