Oswego Lake is in Clackamas County, Oregon, United States, and is completely surrounded by the city of Lake Oswego. Though the lake is naturally occurring, it has been significantly altered because of the concrete dam that has increased its size to 431.7 acres (1.747 km2). The United States Geological Survey records the official name as Lake Oswego and, because of its artificially increased size, classifies it as a reservoir. To distinguish it from the city, however, the lake is usually called Oswego Lake.