Crater Lake occupies a six-mile-wide caldera that was formed nearly seven thousand years ago by the climactic eruption and collapse of Mount Mazama, a twelve thousand-foot-high volcano in the southern Oregon Cascades. The recurrence of smaller eruptions and lava flows produced an emergent cinder cone, known as Wizard Island, a submerged cinder cone called Merriam Cone, and a dome on the floor of the basin. The lake is enclosed by steep caldera walls that ascend from five hundred to two thousand feet above the lake's surface. The lake has a maximum depth of 1,932 feet. (9)

Crater Lake was a popular tourist destination, with cars having to park nose-to-nose between snowbanks at Rim Village in the 1930s. Photo courtesy of the National Park Service

Crater Lake is a closed basin, which means that no permanent streams enter or exit the lake. Water enters the lake as precipitation falling directly on the lake (about 80 percent of the annual water input) and as snowmelt or rain running off the caldera walls. Precipitation occurs mostly as snowfall, which averages about forty-three feet per year. Lake water is lost through seepage (perhaps 50 to nearly 70 percent of the total loss) and evaporation. Since about 1900, the level of the lake has fluctuated nearly sixteen feet, reaching its highest recorded elevation (6,179.3 feet above mean sea level) in 1975 and falling to its lowest recorded elevation (6,163.2 feet above msl) on September 10, 1942. The lake rarely freezes over. Ice cover was reported for 1898 and 1924, and in 1949 an ice layer from two to twelve inches thick covered the lake for three months. The residence time of water in Crater Lake is about one hundred and fifty years, which means that replacement of the lake's entire volume would take that long assuming that water from surface runoff, ground-water seepage, and direct precipitation continued to enter the lake at the current rate. (10)