REPORT:
Debris Flows from Failures of Neoglacial-Age Moraine Dams in the Three Sisters and Mount Jefferson Wilderness Areas, Oregon

The Central Oregon High Cascade Range

Physiography

Geologic and Topographic Setting

In the United States, the Cascade Range extends from northern California to northern Washington. In Oregon, south of Mount Hood, the Cascade Range is 50 to 120 km wide, and is composed primarily of upper Eocene to Quaternary volcanic, volcaniclastic, sedimentary, and igneous intrusive rocks. The crest of the Cascade Range is generally at an altitude of 1,500 to 2,000 m, with several of the high volcanoes exceeding 3,000 m (Callaghan and Buddington, 1938; Sherrod, 1986). The high stratovolcanoes and volcano remnants of the central Oregon Cascade Range include Mount Jefferson, Three Fingered Jack, Mount Washington, the three volcanoes of the Three Sisters, Broken Top, Mount Bachelor, Diamond Peak, Mount Thielsen, and Crater Lake Caldera (Mount Mazama). These are all Pleistocene stratovolcanoes of rhyolitic to basaltic composition and are formed mostly of interlayered thin lava flows and pyroclastic deposits overlying cinder cone cores. Of the large stratovolcanoes, there has been Holocene volcanic activity on the summit and flanks of South Sister and at Mount Bachelor as well as the caldera-forming eruptions of Mount Mazama (Taylor, 1981; Taylor and others, 1987; Scott, 1989). The general conical morphology is best preserved on the volcanoes of Middle Sister, South Sister, and Mount Bachelor; the rest have been deeply eroded by Pleistocene glaciation.

Climate and Vegetation

Western Oregon has a temperate maritime climate that is dominated by winter Pacific frontal systems moving eastward across the State. The Cascade Range is a major orographic barrier that intercepts much of the eastward-flowing moisture. Generally warm and dry summers result from northward expansion of the eastern Pacific high pressure system and diversion of the prevailing westerlies to the north. Consequently, precipitation generally occurs during the winter and is greatest at high altitudes. Annual precipitation is about 3,500 to 4,000 mm at the highest elevations within the Three Sisters and Mount Jefferson Wilderness Areas (Taylor, 1993) and falls mostly as snow. At Crater Lake, 90 percent of the 1,620 mm of annual precipitation falls between October 1 and May 31. Annual precipitation decreases eastward across the Oregon Cascade Range, diminishing from more than 2,500 mm on the western slopes to less than 400 mm within 30 km east of the range crest (Taylor, 1993).

The highest peaks of the central Oregon Cascade Range rise above treeline, which is about 2,200 m above sea level on the north side of Mount Jefferson, about 2,300 m on the north side of South Sister, and 2,500 m on the south side of Broken Top. The tallest trees of the subalpine forests and parks near timberline are mountain hemlock (Tsuga mertensiana) and whitebark pine (Pinus albicaulis).

Glaciers

The central Oregon Cascade Range peaks that presently sustain glaciers or permanent ice masses are, from north to south, Mount Jefferson, Three Fingered Jack, North Sister, Middle Sister, South Sister, and Broken Top. In addition, Mount Bachelor, Diamond Peak, and Mount Thielsen all had small glaciers that persisted until the end of the Little Ice Age in the early 20th century. The Three Sisters Wilderness Area is the most extensively glacierized region of the central Oregon Cascade Range, with 17 named glaciers that presently cover about 7.5 km². In the Three Sisters and Mount Jefferson Wilderness Areas, name assignments to some glaciers have varied over the years. In this report, we use the names on the current U.S. Geological Survey (USGS) topographic quadrangles (1988 provisional editions) and informally use the names "Jack Glacier" and "East Bend Glacier" for the unmapped glaciers or perennial ice masses in the northeast-facing cirques of Three Fingered Jack and Broken Top.