The Yamhill River is an 11-mile tributary of the Willamette River, in the U.S. state of Oregon. Formed by the confluence of the South Yamhill River and the North Yamhill River about 3 miles east of McMinnville, it drains part of the Northern Oregon Coast Range. The river meanders east past Dayton to join the Willamette River at its river mile 55 or river kilomet…The Yamhill River is an 11-mile tributary of the Willamette River, in the U.S. state of Oregon. Formed by the confluence of the South Yamhill River and the North Yamhill River about 3 miles east of McMinnville, it drains part of the Northern Oregon Coast Range. The river meanders east past Dayton to join the Willamette River at its river mile 55 or river kilometer 89, south of Newberg.