The Tualatin River is a tributary of the Willamette River in Oregon in the United States. The river is about 83 miles (134 km) long, and it drains a fertile farming region called the Tualatin Valley southwest and west of Portland at the northwest corner of the Willamette Valley. There are approximately 500,000 people residing on 15 percent of the land in the river's watershed.

Dairy Creek is a 10.55-mile (16.98 km) tributary of the Tualatin River in the U.S. state of Oregon. It begins at the confluence of its east and west forks near the unincorporated community of Schefflin and meanders southeast across the Tualatin Valley to the Tualatin River near Hillsboro, in Washington County. East Fork Dairy Creek begins at 45.788446°N 123.041498°W in Columbia County, slightly north of its border with Washington County, and flows generally south for 22 miles (35 km). West Fork Dairy Creek, also about 22 miles (35 km) long, forms at 45.7553899°N 123.178168°W, near the unincorporated community of Tophill, and flows generally southeast. Before railroads displaced river boats on the Tualatin, some steamships also worked the lower section of Dairy Creek, with plans to go as far up stream as Centerville.