Geological Survey Bulletin 1221-A Rockfalls and Avalanches from Little Tahoma Peak on Mount Rainier, Washington

An interesting side effect of the avalanches was the formation of mudflows; they head at channels in the avalanche deposits and traveled at least as far downstream as White River campground. The mudflows thin downstream from about 6 feet to a few inches. Although they have fairly smooth surfaces, some mudflows a foot or two thick are dotted with many low mounds, a few inches to a foot high and a foot to several feet in diameter (fig. 15). Several mounds were excavated, and a large boulder was found beneath each one. If the mudflows had come to rest on a layer of snow, the mounding and the cracking might have occurred as the supporting snow melted and the mudflow settled over the underlying boulders.

The mudflows downvalley from the tongue of avalanche unit 3 consist of sand and of rock fragments as large as several feet in diameter. In addition, several very large blocks lie on the surface of some of these mudflows. The largest of these is a mass of gray andesite that has maximum dimensions of 9 by 11 by 16 feet; this mass was transported by a mudflow about 4 feet thick. It and other blocks like it are excellent evidence of the remarkable ability of mudflows to transport large rock fragments.

FIGURE 15.—Cracked mounds on surface of mudflow deposit, probably formed as a layer of snow melted under mudflow, permitting mudflow to settle around buried boulders.

The mudflows resulted from the White River spilling over temporary dams of avalanche debris in the narrow gap through the terminal moraine and probably upstream from the moraine as well. As the water ran across the debris it quickly became loaded with loose material and formed masses of bouldery mud that flowed downvalley. The paths of the mudflows through the avalanche deposits were subsequently followed by clearer water that cut flat-bottomed channels and partly eroded the mudflow deposits (fig. 9).