Facts about Crater Lake
Crater Lake is filled with rain and melted snow that fell within the caldera basin. Crater Lake is isolated from surrounding streams and rivers, thus there is no inlet or outlet to the lake. Its primary input is from annual precipitation in the region. Average annual precipitation is 168 cm (66 in); average annual snowfall is 13 m (44 ft). It took approximately 250 years for the lake to fill to today's level (~1,883 m or ~6,178 ft above sea level). The lake maintains its current level because the amount of rain and snowfall equals the evaporation and seepage rate. Lake level has varied only over a range of 5 m (16 ft) in the past 100 years.
Crater Lake is known to be the deepest lake in the United States and the seventh deepest in the world. A maximum lake depth of 608 m (1,996 ft) was recorded by a group of USGS representatives in 1886 using piano wire and lead weight. The maximum depth of 589 m (1,932 ft) was established in 1959 by the USGS using sonar measurement. This depth is referenced at the surface elevation of 1,882 m (6,176 ft). But since its primary input source is dependent upon the climate, lake level is subject to abrupt changes. Crater Lake partially fills the collapsed caldera of the ancient Mount Mazama Volcano. The caldera is a bowl-shape depression of about 1,219 m (4,000 ft) deep.
The maximum depth of Crater Lake recorded at the time of the July 2000 multibeam survey was 594 m ( 1,949 ft). The lake level had an elevation of 1,883 m (6,178 ft) above sea level at the time of the survey. The lake level of Crater Lake fluctuates according to the climate.
The record clarity of Crater Lake was measured at a depth of 41 m (134 ft) in August 1994. The lake clarity is measured with a secchi disk, a black and white disk lowered into the water with a cable. Its exceptional clarity is mainly due to its isolation from streams and rivers. There is no incoming stream to bring any organic materials, sediments, or chemicals to pollute the lake, although natural plankton in the lake and wind-borne pollen have seasonal effects on water clarity. Particulate materials and chemicals are mainly introduced into the lake through precipitation and run-off of the calderal walls. The caldera wall is composed of volcanic rocks that do not react with or dissolve easily in cold water, although warm water escaping from the caldera floor adds a small amount of dissolved solids.
For more information on the physiography of Crater Lake, please see http://www.nps.gov/crla/lake.htm.
For an in depth study of Crater Lake's water supply, please see USGS Open-File Report on Water Balance for Crater Lake, OR.