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 395 acres (1.60 km²). 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).