Historic Resource Study, Crater Lake National Park, Oregon, 1984
V. Geological and Biological Information on Crater Lake Area
C. Description of Lake
The lake? The Sea of Silence? Ah, yes, I had forgotten. . . . But fancy a sea of sapphire set around by a compact circle of the great grizzly rock of Yosemite. . . . The one thing that first strikes you after the color, the blue, blue, even to blackness, with its belt of green clinging to the bastions of the wall, is the silence. . . . 
Although there are larger and deeper lakes in the world, and also other crater lakes–in Asia, South America, Europe, Japan, Australia, Mexico, and Hawaii–Crater Lake is unique for several reasons. First, because the rim walls of volcanic rock rise to such imposing heights and are often reflected in the water to such a degree that reality and the image are one; second, because of the presence of a crater within a “crater”; but primarily because of the puzzling brilliance and depth of its color, a source of extreme wonderment when viewed on a clear day in contrast to the encircling rocky rim of the old volcano with its yellow and rust-colored hues.
Part of the mystery surrounding the color of the water was due to the fact that although from the rim edge the lake surface appears to be of the deepest shade of blue, a glassful taken from the surface shows that the water is actually colorless and remarkably free of sediment, a result of its being fed by direct precipitation rather than by stream flow or seepage. Near the shore it is possible to see the bottom through fifty to seventy-five feet of water, and aquatic moss, which requires sunlight and which can live no deeper than 120 feet in most lakes, has been found growing 425 feet below the surface in Crater Lake.
Studies of the lake water were undertaken, leading scientists to the conclusion that the blueness of Crater Lake is due in great part to the fact that its deep waters remain in a nearly static condition, free of suspended sediment or dissolved matter. The blue and green wave lengths in the sunlight hitting the lake are bounced between and off the water molecules and reflected back upward to the viewer, while the rays of other colors are absorbed. This is a condition especially noticeable in quiet waters of great depth, at a slight distance from the shore, and relatively free of suspended matter. More scientifically, the process is described in this way:
. . the predominant color of the lake is due to multiple scattering of light by the water molecules. Superimposed upon this is the reflection of sky, clouds and crater walls. On a clear day, with rippled surface, the reflection phenomena are entirely submerged by the scattered light. If the sky is overcast, the only light the lake receives is from the clouds; hence the color is predominately gray. With a glassy surface on a clear day, the reflection phenomena may predominate, depending on the position of the observer. The deepening of the color as one approaches the lake is probably due to the fact that at a distance the weaker multiple scattered light, which is the bluest in color, is lost by atmospheric interference and by addition of extraneous light, while at the water surface it has its full effect. . . .
In a word, the blue of Crater Lake and the blue of the sky are due to the selective scattering of rays of light which have been diverted from a straight course by molecules of water, in the one case, and, in the other, by molecules of air . . . . 
Blue is made up of the shortest light waves . . . and it is these that are sent back to the observer due to the extreme depth and purity of the lake water.