Lake Allison

Lake Allison
Location	Willamette Valley, Oregon
Coordinates	45°N 123°W / 45°N 123°W / 45; -123Coordinates: 45°N 123°W / 45°N 123°W / 45; -123
Type	Temporary lake (formed periodically from 12,000 to 15,000 years BP)
Primary inflows	Glacial Lake Missoula
Basin countries	United States
Max. length	111 mi (179 km)
Max. width	31 mi (50 km)
Surface area	3,000 sq mi (7,800 km2)
Average depth	200 ft (61 m)
Max. depth	400 ft (120 m)

Lake Allison was a temporary lake in the Willamette Valley of Oregon, formed periodically by the Missoula Floods from 15,000 to 13,000 BC. The lake is the main cause for the rich and fertile soil that Willamette Valley is now recognized for.

Willamette Valley fertility, like the Palouse silt is in large part due to the largest freshwater flood scientifically documented in history. The ice floods started in Lake Missoula in Montana 12,000 to 15,000 years ago during the and flowed down through eastern Washington State, bringing fertile soil to the valley as it flowed out Columbia River Gorge. The narrows at Kalama, Washington restricted the flow of water, causing it to back up, flooding the Willamette Valley to a depth of 300 or 400 feet above sea level, and reached as far as Eugene, Oregon. The Willamette Valley had multiple floods during the last ice age, possibly reaching 100 floods separated by centuries, 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, Mount Tabor, Rocky Butte, Kelly Butte and Mount Scott would be visible, as would the US Bancorp Tower (536 feet) and the Wells Fargo Center (546 ft). Newberg’s elevation is 175 feet above sea level, Oregon City (138 ft), McMinnville (157 ft), Salem (154 ft), Corvallis (235 ft) and Eugene (430 ft), likely rising above all of them. The lake eventually flowed out and drained, leaving 180 – 200 feet of layered sedimentary soils throughout the Tualatin, Yamhill and Willamette valleys.

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Wikipedia