The Willamette Valley (pronounced /wɨˈlæmɨt/ wi-LAM-it) is the region in northwest Oregon in the United States that surrounds the Willamette River as it proceeds northward from its emergence from mountains near Eugene to its confluence with the Columbia River at Portland. A small part of the Willamette Valley ecoregion is in southwestern Washington, around the city of Vancouver. Being a productive agricultural area, the valley was the destination of choice for the emigrants on the Oregon Trail in the 1840s.

It has formed the cultural and political heart of Oregon since the days of the Oregon Territory, and is home to 70% of Oregon's population. The source of much of the Willamette’s fertility is derived from a series of massive ice age floods that came from Glacial Lake Missoula in Montana and scoured across Eastern Washington, sweeping its topsoil down the Columbia River Gorge. When floodwaters met log-and-ice jams at Kalama, in southwest Washington, the water caused a backup that filled the entire Willamette Valley to a depth of 300-400 feet above current sea level.

Some geologists suggest that the Willamette Valley flooded in this manner multiple times during the last ice age, to depths of 300-400 feet. If 300-400 foot-deep floodwaters descended on the Valley today, in Portland (elevation 20 ft), only the tops of the West Hills, Mt. Tabor, Rocky Butte, Kelley Butte and Mt.

Scott would be visible, as would some of the city’s skyscrapers, such as the US Bancorp Tower (530 feet) and the Wells Fargo Center (540 ft). Newberg’s elevation is 175 feet above sea level, Oregon City (138 ft), McMinnville (157 ft), Salem (154ft), Corvallis (235 ft) and Eugene (430 ft), likely rising above all of them. The lake gradually drained away, leaving layered sedimentary soils on the valley floor to a height of about 180-200 feet above current sea level throughout the Tualatin, Yamhill and Willamette Valleys.

Geologists have come to refer to the resulting lake as Lake Allison. It was named for Oregon State University geologist Ira S. Allison who first described Willamette Silt soil in 1953 and noted its similarity to soils on the floor of former Lake Lewis in Eastern Washington.

Allison also is known for his work in the 1930s documenting the hundreds of non-native boulders (called Erratics) washed down by the floods, rafted on icebergs and deposited on the valley bottom and in a ring around the lower hills surrounding the Willamette Valley. One of the most prominent of these is the Bellevue Erratic, just off Oregon Route 18 west of McMinnville.

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