Upper Klamath Lake (sometimes called Klamath Lake) is a large, shallow freshwater lake east of the Cascade Range in south central Oregon in the United States. The largest freshwater body in Oregon, it is approximately 20 mile (32 km) long and 8 mile (12.9 km) wide and extends northwest from the city of Klamath Falls. It sits at an elevation of 4140 ft (1262 m). The lake depth fluctuates due to regulation of its water supply, ranging from 8 feet (2.5 m) to 60 feet (18 m) deep at average levels. The lake level is kept within 1261 to 1264 m above sea level. It is fed by several sources, including the Williamson River and is drained by the Link River, which issues from the south end of the lake. It is connected by a short channel to the smaller Agency Lake to the north.
Upper Klamath Lake is located in the high desert southernmost part of the state of Oregon. The lake is protected to the northwest by the Cascade Mountains with an arid sagebrush steppe to the east and south. The lake is fed by 17 mineral-rich rivers that deposit an average of 50,000 tons of mineral-rich silt from the surrounding 4,000-square-mile (10,000 km2) volcanic basin, making Upper Klamath Lake one of the richest nutrient traps in the world. The lake waters and its sediments have a high mineral and trace element concentration due to a prehistoric volcanic eruption event (more than 7700 year ago). The event covered the area with millions of tons of mineral ash as far north as the Canadian border.
The regions volcanic legacy is associated with the Pacific Ring of Fire, a geologically active region that experiences large-scale volcanic, tectonic, and glacial events. In the 20th century, the augmentation of nutrients by agricultural runoff from the surrounding farming valley has caused the lake to become hypereutrophic, resulting in blue-green algae blooms over the lake (largely Aphanizomenon flos-aquae). During harvest season, it uses the available nitrogen, creating a massive bloom, choking out competing blooms, so the water's microphyte volume is nearly 100% Aphanizomenon flos-aquae. The lake receives an average 300 days of sunlight per year, which provides a perfect growing environment for this cyanobacterium. The nutrient solution, in the favorable alkalinity of the lake water, provides more than 60 times the nutrients needed for the bloom thus making Upper Klamath Lake the most perfect place for the growth of wild grown, organic Aphanizomenon flos-aquae.