The Geology of Crater Lake National Park, Oregon With a reconnaissance of the Cascade Range southward to Mount Shasta by Howell Williams
Changes of Water Level in Crater Lake
PRESUMABLY Crater Lake began to develop immediately after the formation of the caldera, and at first the water was probably strongly acid on account of the condensation of fumarolic vapors rising through cracks in the floor. Subsequent deepening of the lake was caused partly by direct precipitation of snow and rain and partly by short streams and springs discharging from the walls. Hence, the water is now of exceptional purity. In general, the drainage is outward from the caldera, following the dip of the lavas and interbedded pyroclastic rocks. At several points, however, particularly where lavas rest on glacial debris, the drainage is reversed and enters the lake. Noteworthy examples may be seen on the walls south of Sentinel Point, on Dutton Cliff, and under the topmost flows overlooking Grotto Cove. At these and similar places, copious springs discharge down the caldera walls and often the vegetation near them is unusually lush.
How rapidly the lake level rose there is no means of telling, nor can it be said at what stage the cone of Wizard Island first appeared above the surface. Considering that the submerged walls of the caldera are probably composed in the main of lava flows and that the more porous pyroclastic layers tend to increase in abundance upward, the lake level presumably rose most rapidly during the early stages.
Several years ago, Gordon Hegeness, then on the ranger-naturalist staff, discovered diatomaceous earth on Wizard Island, approximately 50 feet above the surface of the lake. Though there are no corresponding benches on the caldera walls, the occurrence may be considered adequate proof of a former high stand of the water.
During the past forty years, the level of the lake has fallen approximately 13 feet, leaving a series of benches along the shore. According to Diller, there is in addition a seasonal oscillation of about 4 feet, the level being highest early in May when the snows are melting fast, and lowest at the close of the summer. Undoubtedly many springs on the outer slopes of the volcano derive part of their supply from the lake, but it would require long and careful study to determine even approximately how much escapes in this manner. When Diller studied the lake, he estimated that the level during the summer sank 0.0155 foot per day, notwithstanding a small influx, and that the loss by evaporation accounted for 0.0125 foot per day. The remaining fall, 0.003 foot per day, must have been caused by percolation. During the past few years, the amount added to the lake by precipitation and inflow has approximately balanced the loss by evaporation and percolation.