Nivation
When the amount of snow that falls in a region does not all melt during the year the accumulation may result in a permanent snowfield, a mass of ice, or a glacier. In the case of ice, the term glacier is not applied until the mass has reached the moving stage. The transition from snow to ice is brought about largely by the end of winter the usual snow bank is no longer composed of flakes or pellets of snow; instead it is a mass of granular ice to which the term neve is applied.
Where neve fields increase in thickness from year to year eventually the pressure compacts the lower portion of the mass into more or less solid ice; if the mass begins to move the name glacier is applied. Thus a glacier, at least at its source, has a stratification; snow overlies neve which in turn grades downward into more solid ice.
In Crater Lake National Park so little snow lasts through the summer under the present climate that solid ice usually does not form. However, the small amounts of snow that last through the summer as well as the snow that lingers until last June, July or August has been converted into neve. These patches of neve which last into or through the summer exert a limited though definitely noticeable weathering and erosional action.
On nearly level land the geological evidence of neve action (nivation) is perhaps most noticeable. Where neve lasts well into the summer, year after year, the site of the neve is lowered below its surroundings and a small depression is formed. Early in the summer season the accumulation of melt water at the base of the neve during the day is converted into ice at night only to be remelted the next day as more water trickles downward from the overlying neve. This repeated freezing and thawing acomminutes the rock particles. Some water drains downward through the mantle and out of the depression and carries away the finer rock particles. In this manner the depression is enlarged and deepened by the same process of nivation that inaugurated it.
On a sloping terrain nivation often is more active although its evidence may be difficult to distinguish from that of normal erosion. Its results may resemble those of slides and creep phenomena. As nivation continues on a slope the resulting concavity or niche approximates a cirque in appearance although hardly in size. On bedrock nivation operates more slowly than on mantle although the results are similar.
In the higher portions of Crater Lake National Park, nivation is an important and evident geologic process. In the forests and at the base of the talus slopes within the caldera the evidence is not so apparent, but on the treeless expanses there exist many noticeable areas. Several representative examples border the highway from Park Headquarters in the Rim Village near its upper end. These rather flat, treeless expanses are concave upward as a result of greatest activity near the center of the neve field, which in 1949 lasted well into July.
On the back slope of Llao Rock, numerous areas of nivation are easily identified. Some are occupied by neve so late into the summer that practically no vegetation occupies them although along their margins soil and grass cover the pumice at the edge of active nivation. At several localities small serpentine ridges (of) dust-like material were observed on melting of the neve. These ridges, two to four inches high, were also traceable under the neve and marked the egress of streams or rivulets of melt water. Although it is known that rodents dig trails under the snow and neve these were not burrows near the observed ridges. Instead the tiny ridges were composed of water-carried and water-deposited material. In effect they were eskers on a very minute scale.
Numerous forest-free slopes on the higher elevations in the park are the sites of active nivation. Downwards these sites grade into areas which posses similar appearing characteristics and which are believed to have been subject to nivation during past periods of heavier snowfall. On steeper slopes facing both inward and outward in respect to the caldera, nivation is believed to be an active geological agency whose results are largely obscured or exceeded by creep and slide. The small ridges of water-deposited silt are identified as esker-like features produced by sub-neve runoff.
The Frozen Lake
By Franklin C. Potter, Ranger-Naturalist
The biggest news of the year from Crater lake is that its surface froze solid in the winter of 1949. The lake that pamphlets said would never freeze because it was too deep has frozen; and, moreover, stayed frozen for almost three months.
An examination of the winter weather reports since 1926 reveals that the lake had never frozen during that time. However, in The Providence Manual of Information, compiled by the ranger-naturalist staff of 1934, H. H. Waesche reported that the lake was frozen over for two days in 1924. He adds that E. I. Applegate “suspects” that it was frozen at times during the winter of 1897-98 when the temperature at Fort Klamath reached -42° F. Although the lake often has skim ice sometimes over its whole surface, its resistance to freezing is due to the heat reservoir in the immense volume of water.
During the past winter the mean temperatures were lower than ever recorded. December had a mean temperature of 19, January 18, and February 22. The extremes were -9 December, -14 in January, and -8 in February. Considering that only eight out of 17 past winters had weather below zero, it was a cold winter on Mount Mazama.
A limnological survey of Crater Lake revealed that temperature stratification of the lake occurs at about 200 feet. Below that depth the water remains perpetually at 38 degrees. In the upper 100 feet the water temperature varies from 32 to 67, depending upon external factors; the highest temperature is near shallow shores. One reason that the lake fails to warm under the summer sun is a lack of suspended material which would absorb heat and warm the surface water. Because water becomes denser as it cools to 38 in colder weather there is some turnover in the upper layer, the warmer water rising from below. As the surface is cooled below 38 it becomes less dense and the water below imparts heat toward the surface, retarding ice formation. Crater Lake, with its great depth, stores a large amount of heat, even in water of 38 degrees.
This past winter a long period of abnormally low temperatures forced the upper water strata down to 32 degrees and the surface even lower. Heat absorption from the lake by the air was faster than convection of heat from the depths. Ice first appeared around the shoreline and gradually grew towards the center of the lake. After the surface was solid heavy snowfalls deposited four feet of snow on the two inches to one foot of ice. Now that it is known that the lake can freeze under certain conditions, another delicate environmental balance is added to those which determine the character of the mountain and the lake.
