Abstract

Alpine mudflows occurred on the ridges above Steele Creek, St. Elias Range, Yukon, on 11 July 1967, during a rainfall of 2.19 inches in 36 hours. One flow was observed in an area of flat-topped, steep-sided morainal terraces on the lower part of the north valley wall. This mudflow emerged in pulses from a notch at the upper margin of the terrace surface, and formed fronts 6 to 8 ft high. The arcuate fronts broke either to the east or west and the mud flowed across the fan, and through erosional notches to the valley floor. A detailed map and profiles show the topography and distribution of materials.The stream deposited coarse bed load on the alluvial fan on the upper terrace, then cascaded down the notch. Undercutting of the notch caused periodic collapse of the morainal material, which formed temporary dams. Water and finer bed load were impounded and the dam saturated but not overtopped. Saturated material emerged from the notch as viscous, rock-charged mud.Particles in the mudflow range in size from colloids to boulders 13 ft across. Pebbles from a mudflow sample included 70% locally derived lava and 18% granite from the moraine. Material finer than 2 microns was predominantly montmorillonite. Rain and runoff washed fine material from mudflow deposits, leaving a lag deposit similar to alluvial gravel.Botanical evidence indicates that mudflows occur at intervals of decades. The collapse of material from the sides of the steep-walled, high-gradient notch, cut into easily erodable till, controlled the pulsating nature of the observed mudflow.

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