Probing the Depths of Crater Lake: A Century of Scientific Research by Douglas Larson
Other independent researchers also studied Crater Lake from 1978 through 1983. In August and September 1979, Stanford Loeb and John Reuter of the University of California at Davis obtained information on benthic plants inhabiting the lake’s shallow, near-shore waters. A year or so later, David Williams of the USGS and Richard Von Herzen of Woods Hole studied heat flow into the lake and discovered two thermal spring areas on the deep lake floor. They concluded that warm water from the springs ascended through the lake, mixing it from bottom to surface. As the water is heated, it becomes less dense, or lighter. Because the spring waters are warmer than lake waters, they circulate upward (convection), causing the lake to be vertically mixed.
Crater Lake has long been celebrated for its intensely blue color and extraordinary water clarity. On August 27, 1937, Arthur Hasler lowered an eight-inch-diameter white Secchi disk into the lake and observed that it disappeared at a depth of 131 feet; earlier, on August 10 and 19, he had recorded Secchi depths of 118 and 128 feet. These measurements established an important reference point for future scientists who would study the lake’s unusual optical properties. G. E. Hutchinson, renowned limnologist at Yale University, described the lake in 1957 as “almost optically pure.” l re-measured the lake’s water clarity in 1969, obtaining a Secchi depth of 144 feet on July 16. At the time, this depth was thought to be a record Secchi measurement for lakes of the world.
When I took optical measurements almost ten years later in the summer of 1978, however, all of the Secchi-depth readings were less than one hundred feet. The lake’s water clarity had decreased by roughly 25 to 30 percent. This decline in clarity continued through the 1980s. In August 1991, all Secchi readings for the month were less than 85 feet. Once, on August 26, 1991, the disk was visible to a depth of only 67 feet, or about half the distance measured in 1969. Needless to say, these readings were disturbing. Scientists had expected the lake’s rare optical properties to last for “ages” because of its “relative inaccessibility, unique morphometry (bathymetry) and protection by the National Park Service.”