The Waters Of Crater Lake
By J. Stanley Brode, Ranger-Naturalist
The geologist has told the story of Mount Mazama. The mountain has been built up and destroyed, and in that great rent a second volcano appeared. Still more recently a lake has formed. How long has Crater Lake existed? Wizard Island has on its shoulder a mantle of tree life. The trees now there are the first it has borne. By borings and ring counts made by Dr. W. G. Vinal in 1933 the age of the trees was studied and some were found to be over 790 years old. Observations made by scientists and laymen on the slopes of Krakatoa and Katmai indicate that only a few years elapse after eruptions until the volcanic slopes are covered with plant life. This would place the probable cessation of volcanic activity on Wizard Island some 900 to 1200 years ago. Geologists tell us that the rocks do not show characteristics of lava that has flowed into or through water. This leads us to believe that the lake is younger than the island. The lake is probably well under a thousand years old.
In estimating the time required for the lake to fill the caldera or crater to its present level we have four factors to consider: (1) the volume to be filled; (2) the precipitation and drainage into the lake; (3) evaporation from drainage area; (4) seepage through crater walls. From the geological and engineering date we get the following significant figures:
(1) Volume. According to the geologist the hollow in which the lake lies is in the general shape of a truncated cone. Arbitrarily estimating the average height of the rim at 1000 feet above the lake surface with an area within the crater rim of 27 square miles, and the lake surface, with its area of 20 square miles, a plane parallel to the base of the cone, we assume the bottom of the lake to be another plane parallel to the base of the cone and at an average depth of 1500 feet. From this we figure the area of the lake bottom to be nearly 10 square miles. From these figures we estimate the volume of water in the lake at present as about 416 billion cubic feet or 2.82 cubic miles.
(2) Precipitation. The average annual precipitation at the lake is given as 70 inches, most of it in the form of snow, 80 to 100 feet falling at the rim. Over a drainage area of 27 square miles this precipitation would yield 3,885 million cubic feet of water a year, which, if there were no evaporation or seepage to consider, would fill the lake up to its 1908 level in a little over 107 years.
(3) Evaporation. One estimate gives the evaporation from the lake area as 55 inches. While another gives it as 46 inches. Calling the average 50 inches, the effective precipitation available to fill the lake is reduced to 20 inches, which, ignoring seepage, would have filled the lake at a rate of 1,100 million cubic feet a year and would have required 365 years to fill it to its 1908 level.
(4) Seepage. At the present time a balance has been reached between precipitation on the one hand and evaporation and seepage on the other hand, and since the evaporation is approximately 50 inches there remains 20 inches of precipitation lost to be accounted for by seepage. The seepage factor has perhaps been the factor that has checked lake level rise. When the lake began to fill the crater there was doubtless but little seepage. Presuming that over the period of filling the amount of seepage has gone from 0 to 20 inches, we take the average, or 10 inches, as being the average amount of precipitation lost by seepage. That leaves 10 inches as the average effective annual increase deposited in the lake, or 555 million cubic feet a year. At this rate it would have required 730 years to fill the lake, presuming that rainfall and evaporation rates to have averaged as in the past fifty years.
Night On The Lake
By Ernest G. Moll, Ranger-Naturalist
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