100 The Willamette Valley is the section in northwest Oregon in the United States that environs the Willamette River as it proceeds northward from its emergence from mountains near Eugene to its confluence with the Columbia River. One of the most creative agricultural areas of the world, the valley was the destination of choice for the emigrants on the Oregon Trail in the 1840s. It has created the cultural and political heart of Oregon since the days of the Oregon Territory, and is home to nearly 70% of Oregon's population.
The valley may be freely defined as the watershed of the Willamette, bounded on the west by the Coast Ranges, on the east by the Cascade Range. It is enclosed on the south by the Calapooya Mountains, which divide the headwaters of the Willamette from the Umpqua River valley. Because of the differing cultural and political interests, the Portland metropolitan areas, as well as the Tualatin River valley, are often disinclined in the local use of the term. Cities always considered part of the Willamette Valley are Eugene, Corvallis, Albany, and Salem.
The agricultural wealth of the valley is considered to be in no small measure a result of the Missoula Floods, which swamped the valley approximately forty times between 15,000 and 13,000 years ago at the end of the last ice age. The floods were caused by the episodic rupturing of the ice dam of Glacial Lake Missoula, the waters of which swept down the Columbia and flooded the Willamette Valley as far south as Eugene. The floodwaters carried rich volcanic and glacial soil from Eastern Washington, which was deposited across the valley floor when the waters subsided.