Hydrology of Crater, East and Davis Lakes, Oregon by Kenneth N. Phillips
Because of its altitude and position at the crest of the Cascade Range, Crater Lake is in an area whose climate is usually cool and wet from October to May and cool and dry from June to September. Average annual precipitation exceeds 60 inches, and most of it occurs as snowfall. Air temperatures are low throughout the humid period and at night even in summer; ‘but extremely low temperatures seldom occur at the lake or on the slopes of Mount Mazama because of frequent cloud cover and good air drainage. Relative humidity is high except for daytime hours in summer, and evaporation potential is therefore low.
From October to May, moisture-bearing airmasses frequently move up the mountain slopes from the west and southwest. Precipitation in this period is heavy, and the data available are not adequate to show whether it is uniformly distributed or not. Storms from any direction may sweep snow across the crater rim and thence into the lake. Some avalanching into the lake undoubtedly occurs at times from the steeper or less wooded slopes. These factors result in increments of water supply in the lake that may not be accurately indicated by any precipitation gage.
Records of air temperature (table 1) have been collected by the U.S. National Park Service at the sites indicated below, with some periods of incomplete record:
- At Annie Spring (alt 6,016 ft), October 1922 to December 1925.
- At the south rim of Crater Lake (alt 7,086 ft), November 1926 to June 1930.
- At park headquarters (alt 6,475 ft), June 1930 to present. The annual mean temperature is 391F, and from October through May it averages only 32.20F. This long period of coolness affects the temperature of the lake water and lowers the evaporation potential substantially below that of the general area.
The water of Crater Lake is cold. Below a depth of about 200 feet, the temperature approaches 39.2 0F (the temperature of maximum density) at all seasons of the year (Nelson, 1961, p. 36-45). Hence, most of the lake water does not undergo the annual thermal turnovers common to shallow lakes in temperate climates. In summer, the temperature of the surface water rises to about 640F at times.
Contrary to popular belief, ice does form at times on Crater Lake. For instance, the lake was frozen over in 1949, and at that time two men walked across the ice from the south shore to Wizard Island. Again, in January 1962, much of the lake surface was frozen.
Records of precipitation obtained at Crater Lake Weather Station, 2 miles south of the lake rim at altitude 6,475 feet, from 1930 to 1962, are summarized in tables 1 and 2. For the 32 years ending June 30, 1962, the average annual precipitation was 67.4 inches. That value appears to be a little more than the average long-term precipitation, because (1) the average annual precipitation at Grants Pass, 70 miles southwest of Crater Lake, was 30.87 inches for that 32-year period, but only 30.31 inches for the 73-year period 1889-1962, and (2) the lake level was about 8.7 feet higher in July 1962 than it was 32 years earlier-an average rise of more than 3 inches per year.
At the Crater Lake Weather Station the average long-term annual precipitation, adjusted for !a constant lake level, is about 65 inches. In the wettest year ending June 30 of record, 1950-51, the observed precipitation was 93.08 inches, and the lake rose 2.5 feet; in the driest year, 1930-31, the precipitation was 33.34 inches, and the lake fell about the same amount.
The annual precipitation on the lake presumably is not the same as that reported at the weather station. The rain gage collects a sample that is almost infinitesimal as compared to the size of the lake. That sample of precipitation is subject to large and unavoidable errors, such as uneven distribution because of topographic influences and localized wind currents near trees, buildings, or even the precipitation gage itself. Also, the lake basin may at times act as a huge snow trap. For this study, therefore, the precipitation falling on the lake was compared with that observed at the weather station for storm periods in 1961-62 (see p. E15).