Historic Resource Study, Crater Lake National Park, Oregon, 1984
V. Geological and Biological Information on Crater Lake Area
C. Description of Lake
Crater Lake is nestled in a collapse caldera, a large, basin-shaped volcanic depression produced by the failure of the roof of a magma chamber due to removal of the magma by voluminous pyroclastic or lava eruptions or by the subterranean withdrawal of magma.  The lake occupies only about one-eighth of the entire park area, lying in its center at an elevation of about 6,177 feet. Roughly circular in shape, about six miles across at its widest point, and covering twenty square miles, it is rimmed by nearly twenty-six miles of colorful lava cliffs rising from 500 to 2,000 feet above the lake’s surface.
Crater Lake is the deepest lake in the United States, the second deepest in the Western Hemisphere, and the seventh deepest in the world, dropping downward to 1,932 feet just southeast of Merriam Cone. It is eclipsed by five other lakes in the world that exceed 2,000 feet and by Lake Baikal in Soviet Central Asia, the world’s deepest at 5,315 feet. Although not fed by major springs or drained by rivers, the Crater Lake caldera holds a relatively unchanging volume of water- -more than four cubic miles–an amount that usually varies only within five feet of the 6,170-foot mark. This constancy has resulted from a balance between precipitation on the lake’s surface and moisture lost through evaporation and seepage (percolation) through rock strata. Due to its high elevation on the crest of the Cascade Range, Crater Lake receives on the average fifty feet of snow a year, and once more than seventy, with snowfalls covering the park for nearly eight months of the year. Snow melt also contributes to the water level. The water in Crater Lake is always cool, varying from about 64°F. at the surface to a constant 38°F. at depths of 300 feet and beyond. The lake was solidly frozen over in 1949, carrying two to twelve inches of ice.