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U.S. Department of the Interior

Research Brief for Resource Managers

WHAT DO WE KNOW ABOUT IT?

Crater Lake covers the floor of a steep-sided caldera formed 7,700 years ago during the violent volcanic eruption of Mount Mazama followed by collapse of the mountaintop. The average lake level is 1,883 meters above sea level, but for periods of 5 to 10 years lake elevation can vary as much as 5 meters. The lake has a surface area of 54 square kilometers and a maximum depth of 594 meters, making it the seventh deepest lake in the world. The scalloped edge of the caldera is about 8 kilometers in diameter, varying in elevation from 159 to 586 meters above the lake surface. Essentially, the lake rests in a “leaky pot,” losing water only through seepage and evaporation. The lake’s surface elevation rises and drops relative to the amount of snow and rain that falls on its surface.

FIGURE 1. Mean Secchi disk depths in August.

The lake is among the clearest in the world and its waters appear deep blue in color. When sunlight penetrates into the lake, the red and green portions of the light are preferentially absorbed by water and suspended particles. The blue light, which penetrates more deeply, is eventually scattered by water molecules and returns to the lake surface and our eyes. The blueness of the water is greatest when the sun is bright and the particle density is very low. A 20 centimeter black and white disk, known as a Secchi disk, is used to measure the water clarity. In summer, the disk can be seen to a depth of about 30.5 meters, but clarity readings range from under 20 meters to over 40 meters. Changes in water clarity are related to the amount of algae (or phytoplankton) suspended in the lake, as well as particles eroded from the lake shoreline and carried by runoff from numerous small streamlets that enter the lake. Changes in lake water clarity are a natural part of the lake system (Figure 1).

The maximum surface temperatures in August are generally near 18°C. The bottom of the lake has a year-round temperature of about 3.6°C, which is warmed very slightly by the input of geothermal fluids. These fluids also contribute dissolved salts to the lake, creating a much higher salt content than would be expected if direct precipitation were the only source of input.Crater Lake seldom freezes in winter because the great mass of water, although very cold, retains enough heat to keep the surface free of ice. Only during exceptionally cold, windless periods does a thin layer of clear ice cover the lake. This has only happened three times in the last 100 years. During winter, strong winds mix the upper lake waters down to a depth of over 200 meters; however, the deep parts of the lake exchange with the surface only periodically when cold, dense water sinks to the bottom. This process, which appears to be focused along the edges of the lake, forces deep water to “upwell” into the wind-mixed portion of the lake. This physical process occurs annually, but its magnitude varies from year to year due to different weather patterns. Upwelling water is important to the ecology of the lake because it contains nutrients that are important for photosynthesis by algae. The nutrients build up in the deep lake from the decomposition of organic particles sinking to the lake floor; however, the overall concentrations of nutrients such as nitrogen and phosphorus are very low in the lake which is why algal growth is low in the first place. In summer, thermal stratification of the lake results in a warm, less dense surface layer called the epilimnion that floats on top of the deep, cool, dense layer called the hypolimnion (Figure 2). The transitional zone is called the metalimnion.