<Tr> <Th> Surface elevation </Th> <Td> 4,140 feet (1,260 m) </Td> </Tr> <Tr> <Td_colspan="2"> Shore length is not a well - defined measure . </Td> </Tr> <P> Upper Klamath Lake (sometimes called Klamath Lake) (Klamath:? ews, "lake") is a large, shallow freshwater lake east of the Cascade Range in south - central Oregon in the United States . The largest body of fresh water by surface area in Oregon, it is approximately 25 miles (40 km) long and 8 miles (13 km) wide and extends northwest from the city of Klamath Falls . It sits at an average elevation of 4,140 feet (1,260 m). </P> <P> In the early 20th century, most of the wetlands and marshes surrounding the lake were drained for agricultural purposes, damaging the larger habitat . The lake's water has been used to support the federal Klamath Project for irrigation by local farmers for a century . The lake depth fluctuates due to regulation of its water supply, averaging between 8 to 50 feet (2.4 to 15.2 m) deep . The lake level is kept within 4,137 to 4,143 ft (1,261 to 1,263 m) above sea level to ensure a viable fishery in the lake and to protect coho salmon in the Klamath River below the lake . </P>

What is the largest natural body of water in oregon