Debris flows from failures of neoglacial-age moraine dams in the Three Sisters and Mount Jefferson Wilderness areas, Oregon

Abstract

Cascade Range alpine glaciers have shrunk substantially as average annual temperature has risen 0.5 to 2 degrees Celsius since culmination of the Little Ice Age in the mid- to late 1800's. In recently deglaciated areas inthe Cascade Range, hundreds of lakes have formed. Most of these newly formed lakes are partly or entirely bound by bedrock rims and are stable, but at least 30 are dammed by unconsolidated moraines that are susceptible to breaching. The highest concentration of lakes dammed by Neoglacial moraines in the conterminous United States is in the Mount Jefferson and Three Sisters Wilderness Areas in central Oregon, where there are currently eight moraine-dammed lakes. The largest lake, Carver Lake on South Sister, has a volume of almost 1 million cubic meters. Most of these lakes formed between 1920 and 1940 during a period of substantial warming and glacier retreat. In the Mount Jefferson and Three Sisters Wilderness Areas, there have been 11 debris flows from 4 complete and 7 partial emptyings of moraine-dammed lakes. Most of these breaches occurred between 1930 and 1950, but some were as recent as the 1970's. All moraine-dam breaches in the Three Sisters and Mount Jefferson Wilderness Areas occurred during the melt season (July-October), usually during periods of warm or rainy weather. Many breaches were probably a result of erosion of the steep outlet channels, triggered by unusually large discharges caused by (1) waves generated by rockfalls or ice avalanches into the lake or (2) increased lake outflow caused by precipitation and melting snow and ice. Water flows from breached moraine dams rapidly evolved into debris flows that traveled as far as 9 kilometers before stopping or evolving into sediment-laden water flows. Peak discharges of at least four of the flows exceeded 300 cubic meters per second. Flows from breached morainal dams transformed from clear water at the outlet into debris flows within 500 meters of the breaches by incorporating large volumes of loose Neoglacial till and outwash from the moraines and proglacial outwash. For the two largest lake releases, the volume of sediment eroded near the outlet exceeded 25 percent of the total volume of water released. Morphological evidence indicates that sediment was introduced into flows by bank collapse and channel incision. Indirect discharge estimates (primarily by a critical-depth procedure) show that peak discharges increased in erosional reaches; in one instance by more than a factor of four. Erosion and sediment entrainment was restricted to reaches with slopes that exceeded 8°, and deposition occurred in reaches with slopes less than 18°. Several moraine-dammed lakes still exist, and some pose downstream hazards. Two of the lakes are remnants of previously larger lakes that have partially breached their moraine dams. Five lakes in the Three Sisters and Mount Jefferson Wilderness Areas are impounded by Neoglacial moraines that have not been breached. Qualitative assessments of downstream hazards from moraine-dammed lakes are possible on the basis of the topographic setting of the lake and downstream channel conditions. Quantitative assessment of the likelihood of breaching or the magnitude of downstream flows is difficult because of the variety of mechanisms that trigger breaches, the sensitivity of outflow hydrographs to breach shape and erosion rate, and the large uncertainty of downstream flow characteristics.