Kautz Creek Mudflow, October 2-3, 1947

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The youngest deposits of the lahar assemblage in the Kautz Creek valley were formed during the night of October 2-3, 1947, and during the early morning hours of October 3. Details of the events that preceded and accompanied the lahars are largely taken from reports by R.K. Grater (1948; unpub. data, October 28, 1947, in files of the National Park Service) and from an unpublished, undated report by C.E. Erdmann and Arthur Johnson of the U.S. Geological Survey, also in the files of the National Park Service.

According to Erdmann and Johnson, rain started to fall a few hours after midnight on October 1 and reached its greatest intensity between 5 and 9 a.m. on October 2. A Total of 5.85 inches fell at the Ranger Station at Paradise Park between 4:30 p.m. October 1 and the same time the next day, but very little fell during the early hours of October 3. During the downpour, a cloudburst evidently occurred in the upper part of the Kautz Creek drainage basin, and an additional amount of water may have been contributed by a "glacier outburst flood" from Kautz Glacier (Richardson, 1968, p.D83). The discharge of Kautz Creek increased greatly during the daylight hours of October 2, and between 7 and 8 p.m., high water washed out the entrance road bridge over the creek. The first lahar occurred between 10 and 11 p.m. and was heralded by a dull roar and vibration of the ground as it advanced down the valley. A succession of lahars flowed downvalley during the night, and the last occurred at 8 a.m. the next morning. The lahars were described by eyewitnesses as having the consistency of wet concrete, carrying along vegetation and boulders as large as 13 feet in diameter.

After the lahars ceased, Kautz Creek was flowing in a trench 30-75 feet deep and 100-200 feet wide south of the "box canyon." ... (the "box canyon" of the Kautz, which is a bedrock-walled constriction in the valley southeast of Pearl Falls and 5 miles northeast of the Entrance Road. The "box canyon" is about a quarter of a mile long and 250-300 feet wide.) ... How much of this trench existed before the lahars occurred is not clear; Grater (1948, p.279) stated that "an entirely new stream channel was cut, with the old bed of Kautz Creek left high on the side of the canyon." The topography of the valley floor suggests, however, that the present trench could have been localized along a preexisting channel. Remnants of the 1947 lahars form discontinuous narrow strips along the edges of the trench, and although they locally extend beyond the trench in lobes, they do not veneer wide areas in the upper part of the valley. Their distribution suggests that the lahars were largely confined to the trench and that the trench either predates them or was formed contemporaneously. The narrow strips of laharic deposits are generally less than 10 feet thick, but near the south end of the trench they are as much as 18 feet thick.

Even though the present trench may have been localized along a former course of Kautz Creek, that stream course undoubtedly was cut down and widened at some time on October 2 or 3. Evidence from the lahar assemblages exposed in the Kautz Creek valley and in other valleys at Mount Rainier indicates that lahars themselves are not effective erosional processes at the gradients found on most valley floors below the glaciers. If this is true, downcutting of the trench probably was accomplished not by the lahars themselves, but by floodwaters.

During the morning of October 3, according to the report of Erdmann and Johnson, the valley floor downstream from the trench aggraded about 28 feet where it is crossed by the entrance road. The area of aggradation forms a fan whose base, along the Nisqually River, has a breadth of about 2,500 feet. The area narrows northward to 500 feet or less and apexes at the mouth of the trench. Although the fan in the lower part of the valley probably contains most of the coarse debris transported on October 2 and 3, much of the fine material was carried downstream by the Nisqually River and was deposited on the floor of Alder Reservoir. Grater (1948) estimated the total volume of the lahars and flood deposits to be at least 50 million cubic yards.

During a subsequent investigation of the upper Kautz Creek valley, R.K. Grater (unpub. data, 1947; 1948) found that the lower 1-mile segment of Kautz Glacier had largely disappeared, and a gorge 300 feet deep and as much as 900 feet wide had been cut into the ice. Grater inferred that the gorge had been cut by excessively high runoff, aided by toppling of ice masses as the glacier was undercut. Blocks of ice, accompanied by masses of glacial drift that slid from the precipitous canyon walls, created temporary dams in the "box canyon," the failure of which caused floods of water and masses of saturated debris to flow down the valley. Repetition of sliding and damming caused successive lahars and floods to surge down the valley. The trench of Kautz Creek probably was cut by the floods, and this erosion itself may have created lahars as the undercut sides of the trench toppled into the floodwaters.

Samples of some of the fluvial gravels and lahars deposited on October 3, 1947, were obtained from an outcrop in the bank of the Nisqually River along the south edge of the fan. ... X-ray analysis of a sample of one of the lahars indicated that the silt and clay fractions consist almost wholly of plagioclase feldspar.