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Lake Allison
Location Willamette Valley, Oregon
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)[1]
Average depth 200 ft (61 m)[2]
Max. depth 400 ft (120 m)[3]

Lake Allison was a glacial backwater floodplain lake created from naturally occurring ice dams in the Willamette Valley of Oregon. The lake is the main cause for the rich and fertile soil that Willamette Valley is now recognized for.

Contents

History

 Figure showing topographic maps of Washington and northern Oregon with the lowlands flooded by the Missoula Floods marked.
Location of the former Lake Allison.

Willamette Valley fertility, like the Palouse silt[4] is in large part due to the largest freshwater flood scientifically documented[5] in history. The ice floods started in Lake Missoula in Montana 12,000 to 15,000 years ago during the Pleistocene[6] and flowed down through eastern Washington State, bringing fertile soil to the valley as it flowed out Columbia River Gorge. Natural ice dams at Kalama, in southwest Washington caused a backup that filled the Willamette Valley to a depth of 300 or 400 feet above sea level, and reached as far as Eugene, Oregon.[7][8] The Willamette Valley had multiple floods during the last ice age, possibly reaching 100 floods separated by centuries,[9] to depths of 300-400 feet.[10][11] 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, Kelley Butte and Mount Scott) would be visible,[12] 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 (154ft), 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.[13][14]

Name

Geologists named the lake after Oregon State University geologist Ira S. Allison. Among other things, he was the first person to identify and correlate Willamette silt soil in 1953 with soils at the former lakebed of Lake Lewis in Eastern Washington. Ira Allison also documented hundreds of non-native boulders (also known as glacial erratics), in the 1930's, which were transported down the river by the floods on icebergs and left a ring around the lower hills surrounding the Willamette Valley. The most notable of these is the Bellevue Erratic, off Highway 18, west of McMinnville.[15]

Notes

  1. ^ http://www.columbiariverimages.com/Regions/Places/missoula_floods.html
  2. ^ http://www.columbiariverimages.com/Regions/Places/missoula_floods.html
  3. ^ http://www.columbiariverimages.com/Regions/Places/missoula_floods.html
  4. ^ http://www.columbiariverimages.com/Regions/Places/missoula_floods.html
  5. ^ http://www.waymarking.com/waymarks/WM19FR
  6. ^ http://depts.clackamas.cc.or.us/science/physci/geology/G145/lectures/wil%20valley.htm
  7. ^ Cataclysms on the Columbia, by John Elliott Allen and Marjorie Burns with Sam C. Sargent, 1986. Pages 175-189
  8. ^ http://www.waymarking.com/waymarks/WM19FR
  9. ^ http://www.waymarking.com/waymarks/WM19FR
  10. ^ Cataclysms on the Columbia, by John Elliott Allen and Marjorie Burns with Sam C. Sargent, 1986. Pages 175-189
  11. ^ Geology of Oregon, by Elizabeth L. Orr, William N. Orr and Ewart M. Baldwin, 1964. Pages 211-214
  12. ^ Geology of Oregon, by Elizabeth L. Orr, William N. Orr and Ewart M. Baldwin, 1964. Pages 211-214
  13. ^ Geology of Oregon, by Elizabeth L. Orr, William N. Orr and Ewart M. Baldwin, 1964. Pages 211-214
  14. ^ http://www.columbiariverimages.com/Regions/Places/missoula_floods.html
  15. ^ Cataclysms on the Columbia, by John Elliott Allen and Marjorie Burns with Sam C. Sargent, 1986. Pages 175-189

References

External links

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