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Oswego Lake
Oswego Lake is a lake in Clackamas County, Oregon that is completely surrounded by the city of Lake Oswego. Though the lake is naturally occurring (a former channel of the Tualatin River), it has been significantly altered because of the concrete dam that has increased its size to . Officially, the United States Geological Survey records the official name as ''Lake Oswego'' and classifies it as a reservoir (due to its artificially increased size).〔(【引用サイトリンク】title=Feature Detail Report for: Lake Oswego )〕 To distinguish it from the city, however, the lake is usually called ''Oswego Lake''.
==Geologic history==
The lake is a former channel of the Tualatin River, carved in basalt to the Willamette River. Eventually, the river changed course and abandoned the Oswego route.
About 13,000 to 15,000 years ago, the ice dam that contained Glacial Lake Missoula ruptured, resulting in the Missoula Floods, which backed the Columbia River up the Willamette River. The flooding created an underwater vortex called a kolk, which scoured out and enlarged the old Oswego channel, creating a natural lake. The rocks and boulders were flung by the kolk up to a mile away to present-day Durham and Tualatin, where they were quarried for many years before the site was converted to the Bridgeport Village shopping center.〔〔(【引用サイトリンク】title=Oswego (Sucker) Lake, Lake Oswego, Oregon Waymark )〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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