Crater Lake is a volcanic crater lake in south-central Oregon in the western United States. It is the main feature of Crater Lake National Park and is famous for its deep blue color and water clarity. The lake partly fills a 2,148-foot-deep (655 m) caldera that was formed around 7,700 (± 150) years agoby the collapse of the volcano Mount Mazama. There are no rivers flowing into or out of the lake; the evaporation is compensated for by rain and snowfall at a rate such that the total amount of water is replaced every 250 years. With a depth of 1,949 feet (594 m), the lake is the deepest in the United States. In the world, it ranks ninth for maximum depth, and third for mean (average) depth.
Crater Lake features two small islands. Wizard Island, located near the western shore of the lake, is a cinder cone approximately 316 acres (128 ha) in size. Phantom Ship, a natural rock pillar, is located near the southern shore.
Since 2002, one of Oregon's regular-issue license plate designs has featured Crater Lake and a one-time plate surcharge is used to support the operation of Crater Lake National Park. The commemorative Oregon State Quarter, which was released by the United States Mint in 2005, features an image of Crater Lake on its reverse.
The lake and surrounding park areas offer many recreational activities including hiking, biking, snowshoeing, fishing, and cross-country skiing are available, and during the summer, campgrounds and lodges at Crater Lake are open to visitors.
Since the collapse of Mount Mazama due to a volcanic eruption formed Crater Lake, no fish inhabited the lake until William G. Steel decided to stock it in 1888 to allow for fishing. Regular stocking continued until 1941, when it was evident that the fish could maintain a stable population without outside interference. Six species of fish were originally stocked, but only two species have survived: Kokanee Salmon and Rainbow Trout with Kokanee being the most plentiful. Fishing in Crater Lake is promoted because the fish species are not indigenous to the lake.
Crater Lake is also known for the "Old Man of the Lake", a full-sized tree which is now a log that has been bobbing vertically in the lake for over a century. The low temperature of the water has slowed the decomposition of the wood, hence its longevity.
In 1987, scientists sent a submersible down to the depths of Crater Lake to obtain more information about the geology at the bottom of the lake, and inspect moss samples found in moss beds as deep as 600 feet (180 m).
Due to several unique factors, mainly that the lake has no inlets or tributaries, the waters of Crater Lake are some of the purest in the world because of the absence of pollutants. Clarity readings from a Secchi disk have consistently been measured as being 120 ft (37 m), which is very clear for any natural body of water. In 1997, scientists recorded a record clarity of 142 ft (43 m).
The lake has relatively high levels of dissolved salts, total alkalinity, and conductivity. The average pH has generally ranged between 7 and 8.