In 1886 a group of men representing the United States Geological Survey measured the depth of Crater Lake in several places. Using piano wire and a lead weight, they determined that the greatest depth of Crater Lake to be 1,996 feet. Seventy-three years later, another U.S.G.S. survey corrected the earlier measurement by using sonar and established the greatest depth at 1,932 feet. This depth is referenced against a surface elevation of 6,176 feet. Because the lake loses water through evaporation and seepage, there are times when the lake depth is less than the 1959 calculation. Since inputs vary from year to year, there must also be periods when the 1,932-foot calculation is exceeded.

The primary input for Crater Lake is the annual precipitation the region receives. This is close to 69 inches on average, as measured at Park Headquarters. The lake level rises from October to April because input exceeds output, as seen in Fig. 1. As precipitation lessens in late spring, the lake's level stabilizes until mid June. This is due to the balance among evaporation, seepage, reduced precipitation, and run-off from melting snow. For the rest of the summer, the lake level falls at an average daily rate of .675 centimeters per day, or about 25 hundredths of an inch. The lake is usually at its lowest level at the end of September. For the lake to return to the same level year after year, the input as measured at Park Headquarters must be 66.9 inches. This amount is dose to the long term average measured at this site.

Figure 1. Annual cycle of water level. Fifteen-year composite of 1996-1971, 1975-1976, 1979-1985. Units: Elevation in meters minus 1859.28m.

The lowest lake level was recorded on September 10, 1942, when the lake dropped to a surface elevation of 6,163.20 feet. This reading is related to low precipitation amounts observed regionally during the 1930s. In 1975, the lake level reached a historic high when it rose to a level of 6,17934 feet. There is some evidence that the lake may never get much higher than this 1975 measurement. Lichen stains on rocks near shoreline indicate that the water may never have been above 6,180.50 feet. This evidence is also consistent with observations that live and dead trees have rooted just a few feet above the observed lake level maximum.

Since Crater Lake is thought to have no other significant input, lake level is subject to abrupt changes year to year when snowfall amounts vary. For example, the lake level rose two and a half feet between 1951 and 1952. Conversely, it fell 3.40 feet between 1976 and 1977. When snowfall accumulation reaches levels near historic averages, little change occurs to the lake level from year to year. As noted earlier, the range of lake level measurements has varied some 16.14 feet over the past century. The 30 year average for the lake's surface elevation is 6,175 feet. This is slightly higher that the average for the period of 1907 to 1988, which is 6,170 feet; the latter elevation reflects the low water level observed in the 1930s and early 1940s.

During the summer of 1991, the lake's surface elevation was estimated to be 6,170.80 feet on July 31. Assuming that there was no significant precipitation through September, the lake level might have been near 6,170 feet on September 30. This would be the lowest level since the early 1950s. Like the low lake levels recorded earlier this century, the present trend seems related to less than average precipitation amounts beginning in 1985.

A hundred years of lake measurements have taught us that we should not assume that the level of Crater Lake does not change. Thirty-year or even hundred-year averages can be very misleading. What we observe one year or over one decade is no indication of what can happen the next. Furthermore, since lake level changes are related to regional climatic events, it is impossible to forecast the next season's lake level accurately. When the figure of 1,932 feet is cited as Crater Lake's depth, this is only one observation made at a given time during the recent past. As with any dynamic system, the depth of Crater Lake and its elevation above sea level is never a fixed value. Each rise and fall is a response to forces imposed by nature from outside the caldera.