The Klamath river begins below Upper Klamath Lake in Southern Oregon and flows about 300 miles through Northern California until draining into the Pacific ocean. Upper Klamath Lake is the largest freshwater lake in Oregon and was established by the late Pliocene period. The lake fills a graben that is thousands of meters deep. The Klamath Basin lies on the edge of the Basin and Range province and is adjacent to the High Cascades of Southern Oregon.