Elsevier

Geomorphology

Volume 39, Issues 3–4, August 2001, Pages 111-129
Geomorphology

Formation and failure of volcanic debris dams in the Chakachatna River valley associated with eruptions of the Spurr volcanic complex, Alaska

https://doi.org/10.1016/S0169-555X(00)00097-0Get rights and content

Abstract

The formation of lahars and a debris avalanche during Holocene eruptions of the Spurr volcanic complex in south-central Alaska have led to the development of volcanic debris dams in the Chakachatna River valley. Debris dams composed of lahar and debris-avalanche deposits formed at least five times in the last 8000–10,000 years and most recently during eruptions of Crater Peak vent in 1953 and 1992. Water impounded by a large debris avalanche of early Holocene (?) age may have destabilized an upstream glacier-dammed lake causing a catastrophic flood on the Chakachatna River. A large alluvial fan just downstream of the debris-avalanche deposit is strewn with boulders and blocks and is probably the deposit generated by this flood. Application of a physically based dam-break model yields estimates of peak discharge (Qp) attained during failure of the debris-avalanche dam in the range 104<Qp<106 m3 s−1 for plausible breach erosion rates of 10–100 m h−1. Smaller, short-lived, lahar dams that formed during historical eruptions in 1953, and 1992, impounded smaller lakes in the upper Chakachatna River valley and peak flows attained during failure of these volcanic debris dams were in the range 103<Qp<104 m3 s−1 for plausible breach erosion rates.

Volcanic debris dams have formed at other volcanoes in the Cook Inlet region, Aleutian arc, and Wrangell Mountains but apparently did not fail rapidly or result in large or catastrophic outflows. Steep valley topography and frequent eruptions at volcanoes in this region make for significant hazards associated with the formation and failure of volcanic debris dams.

Introduction

High relief, an extensive cover of snow and ice, and an ubiquitous mantle of loose, unconsolidated volcaniclastic debris are characteristic of volcanic regions around the world, especially the great volcanic arcs of the Pacific Rim. Pyroclastic eruptions from volcanoes in these areas almost always result in the production of large amounts of water and formation of voluminous lahar flows. The lahar flows rapidly enter lowland areas and valleys beyond the volcano where they may temporarily or permanently alter the valley bottom topography and disrupt the hydrologic regime of streams and rivers. A specific type of disruption is the formation of lahar dams. Lahar dams often develop on the distal flanks of volcanoes in areas where narrow valleys confine lahars and their deposits form extensive blockages across main streams and tributaries. Lahar dams can form in almost any volcanic region, but are most common in areas of high relief where valley topography around the volcano is steep and pyroclastic eruptions are frequent.

Another common but less frequently occurring phenomenon at many stratovolcanoes is the generation of debris avalanches associated with large-scale collapse of a portion of the volcano flank. Typical volcanic flank collapses involve 0.1 to 1 km3 or more of rock debris that may mechanically transform to a debris avalanche and sometimes to lahar Siebert, 1996, Vallance and Scott, 1997, Iverson et al., 1998. Debris avalanches and lahars can travel several tens of kilometers beyond their source areas and may inundate significant amounts of bottomland area in valleys and drainages on the volcano and beyond Pierson et al., 1990, Vallance and Scott, 1997, Iverson et al., 1998. One common consequence of debris-avalanche emplacement is the formation of debris dams where the avalanche debris blocks a river valley Youd et al., 1981, Meyer et al., 1985, Glicken et al., 1989, Costa and Schuster, 1991. In general, the potential for debris avalanche and subsequent debris dam formation is greater at active, glaciated stratovolcanoes where the combination of high relief, weak hydrothermally altered rocks in the edifice, and tectonic setting promote flank instability.

Natural dams of all types are capable of impounding substantial amounts of water, and floods resulting from natural dam failures often have peak outflows that rival the peak flows of the largest known rainfall floods Baker and Costa, 1987, Costa and Schuster, 1988, Costa and Schuster, 1991. Natural debris dams are common in mountainous environments, especially in areas with narrow, steep valleys and weak bedrock or hillslope materials. Landslide dams, the mechanisms of natural dam failure, and associated dam-break floods are discussed in Costa and Schuster, 1988, Costa and Schuster, 1991, Clague and Evans (1994), Fread (1996) and Walder and O'Connor (1997). The formation and failure of volcanic debris dams associated with eruptive activity in Alaska is not widely known or discussed in the literature but has occurred at three of five historically active volcanoes in the Cook Inlet region of south-central Alaska (Fig. 1), at several volcanoes on the Alaska Peninsula (Fig. 1), and has been well documented at Mt. St. Helens, Washington Youd et al., 1981, Meyer et al., 1985, Glicken et al., 1989. The formation of volcaniclastic debris dams is often a direct consequence of pyroclastic eruptions at snow and ice clad stratovolcanoes worldwide Knott and Smith, 1980, Silva et al., 1982, Costa and Schuster, 1991. In this paper, evidence for several episodes of debris dam formation associated with eruptions of the Spurr volcanic complex (Fig. 1) are described. Dam failure conditions and peak discharge are estimated with a dam-break model and regression equations that relate peak discharge to dam and reservoir characteristics Walder and O'Connor, 1997, Manville et al., 1999.

Section snippets

Physical setting and the Spurr volcanic complex

The Spurr volcanic complex (SVC) is located about 100 km west of Anchorage, Alaska, within the Tordrillo Mountains on the north side of Cook Inlet (Fig. 1). The SVC consists of ancestral Mt. Spurr Volcano, an andesitic lava dome complex that forms present day Mt. Spurr, and Crater Peak, a flank vent on the south side of Mt. Spurr Volcano (Fig. 2). Crater Peak was the site of the most recent eruptive activity of the SVC in 1953 and 1992 and the active vent throughout most of the Holocene Riehle,

Holocene eruptive history

The major volcanic event of the past 10,000 years at Mt. Spurr Volcano was the formation of the present caldera during a major flank collapse that either caused or resulted from a large eruption. An extensive debris-avalanche deposit and an overlying sequence of block-and-ash-flow deposits are preserved on the proximal southern flank of the volcano. These deposits and the caldera structure itself are the only known evidence of this major eruption. The age of the debris avalanche and associated

Flank collapse and debris avalanche

A distinguishing characteristic of Spurr Volcano is a 6-km diameter, circular caldera structure that encloses the summit dome complex of Mt. Spurr (Fig. 2). This feature and others like it are diagnostic of large-scale failures of a volcanic edifice Siebert, 1984, Siebert, 1996. Debris-avalanche deposits generated by flank collapse typically extend from a breach in the collapse caldera to the distal flanks of the volcano and beyond. The southern portion of the caldera of Spurr Volcano has a

Downstream effects

Unstable volcanic debris dams have probably formed in the upper Chakachatna River valley at least five times in the past 8000–10,000 years including two episodes of dam formation and failure during historical eruptions in 1953 and 1992. The lakes impounded by the debris dams alone would have generated large floods on the Chakachatna River when the debris dams failed. The largest lakes impounded by the debris dams had volumes in excess of 108 m3 and their presence in upper Chakachatna River

Volcanic debris dams at other Alaskan volcanoes

Lahar generation has been a common phenomenon during Holocene eruptions at many volcanoes in the Aleutian arc. Most of the valleys on or nearby these volcanoes have been inundated by lahars many times since they were deglaciated at the end of the Pleistocene Epoch. For example, during Holocene eruptions of Redoubt Volcano, lahars flowed up the Lake Fork of the Crescent River forming a lahar dam that now impounds Crescent Lake Riehle et al., 1981, Begét and Nye, 1994 (Fig. 1). The Crescent Lake

Conclusions

Future pyroclastic eruptions of Aleutian arc volcanoes will cause lahars that will likely form natural dams. Steep, narrow valleys that are susceptible to lahar inundation are common at many Aleutian arc volcanoes, especially those in the Cook Inlet region. Such valleys are easily blocked by lahar deposits and impoundments of various sizes could develop. Additionally, large-scale flank collapses that result in debris-avalanche formation could generate debris dams of great size that would

Acknowledgements

The thoughtful reviews and comments by Joe Walder, Jim O'Connor, Ellen Wohl, Cathy Connor, and Chris Nye are greatly appreciated.

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