Upper Klamath Lake is the largest freshwater lake in Oregon, and one of the largest in the nation. The lake lies east of the Cascade Mountains. Although the algae that grow in its waters are generally harmless, they give the lake the impression of pollution and recreation on the lake is limited as a result. Efforts are underway to reduce the amount of nutrients, due primarily to runoff from agricultural lands at the north end of the lake. Klamath Falls
is located at the south end of the lake.