Geological Survey Professional Paper 3 The Geology and Petrography of Crater Lake National Park

CRATER LAKE.
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INFLOW OF CRATER LAKE.

The drainage area within Mount Mazama is very small as compared with the size of the lake. It is circumscribed by the crest of the rim and limited to its inner slope, with an area, according to Mr. E. C. Barnard, of 27.48 square miles. As the area of the lake, including Wizard Island, is 21.30 square miles, the drainage area encircling the lake is 7.06 square miles—that is, less than one-third of the size of the lake itself. The prevailing southerly and southwesterly winds of winter, when the precipitation is largest and is in the form of snow, greatly increases the catchment over the normal precipitation by drifting snow over the southern rim. It hangs upon the inner slope and furnishes many avalanches to the lake during winter as well as many small streams in summer. Sixty-three small streams were closely estimated or measured by running them over a weir at the end of a trough, and it was found that during the time we were there they added to the lake 10.754 cubic feet per second, or 929,145.6 cubic feet per day. The area of the water surface of the lake is approximately 20.42 square miles, and the water added to the lake in a day, although it seems large in the streams, would raise its surface only 0.00002 foot, an amount which is not appreciable.

OUTLET OF CRATER LAKE.

Crater Lake has no visible outlet, nor any other reaching the surface directly within a few miles, for if such were the case the water would issue with great force on account of the pressure. If there are outlets at lower levels the route to the opening must be long and tortuous, so that the friction completely overcomes the pressure and allows the water to issue as from an ordinary spring.

The annual precipitation in the region, roughly approximated years ago by Mr. Gannett and republished by Mr. Russell, was given as from 60 to 70 inches, while the annual evaporation, by the same authority, is 40 to 50 inches.

We may get a suggestion as to the precipitation in the region from the snow line marked by moss on the trees. On the southwest portion of the rim the snow line ranges from 5 to 20 feet above the ground. The variation is due to drifting. On Wizard Island among the trees the wind has much less effect, and the snow line is sharply marked 12 feet from the ground. This probably represents a fair approximation to the average depth of snow. Most of the precipitation is snow, although there is some rain. There is considerable snow properly included in the precipitation estimate, but not represented in the snow line.

Measurements were made in a 12-foot snow bank near camp to determine the water equivalent for such a bank of snow. The average of a number of trials at different levels, which differed but little among themselves, was that 37 inches of snow yielded 22 inches of water. The snow was probably more compact than that of the island, but it was not saturated with water, for we failed entirely to get water from snow holes, as we had done in previous years. The snow was dry. Twelve feet of such snow would equal about 85 inches of water, and it is believed that the average annual precipitation for that region is nearer 80 than 70 inches.

The peculiar position of the caldera on the summit of the broad range greatly increases the catchment of the basin, for the winter storms from the south and southwest drift vast quantities of snow from the gentle outer slopes of the rim across the crest, to lodge in great banks on the inside. It seems evident, there ore, that much water must escape from this closely landlocked lake by percolation besides that which leaves by evaporation.

It was for the purpose of getting, if possible, an approximate measurement of the amount thus lost by percolation that observations were made last summer of the evaporation and inflow at the same time that the sinking of the lake surface was measured. From these observations it appears that while the lake sank at the rate of 0.0155 foot per day, notwithstanding a small influx, the loss by evaporation was only 0.0125 foot per day. In other words, an amount equal to about one-fourth of that lost by evaporation escapes by percolation through the porous base of Mount Mazama.

The walls of the caldera inclosing the lake are made up of alternating sheets of lava dipping away from the lake practically in all directions, and are so porous as to afford easy passage for much water. Springs are abundant and remarkable in size, especially on the southeast side, along a fault which forms a bluff extending from Modoc Point by Fort Klamath far up directly toward Crater Lake. It is very probable that this fault, cutting the old lavas of Mount Mazama, affords an outlet for much of the water that percolates through that portion of the rim. Much of this spring water appears to be appreciably warmer than that of the main body of Crater Lake.