Wil·lam·ette River (w?-l?m??t) A river, about 480 km (300 mi) long, of northwest Oregon flowing generally northward to the Columbia River near Portland. The Willamette Valley, a fertile agricultural region, was settled in the 1830s by pioneers traveling west along the Oregon Trail.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.