PerspectiveVOLCANOLOGY

Mount St. Helens at 40

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Science  15 May 2020:
Vol. 368, Issue 6492, pp. 704-705
DOI: 10.1126/science.abb4120

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Sediment is trapped behind a retention structure (out of view) on the North Fork Toutle River. Mount St. Helens is visible in the background. Since 1998, some of the flushed sediment has bypassed the structure, increasing flood hazards downstream.

PHOTO: ADAM MOSBRUCKER/USGS

American baseball legend Yogi Berra famously quipped “It ain't over till it's over.” That tautological phrase is apt for volcanology, especially with respect to eruptions that pummel landscapes with fragmental debris. Indeed, after such eruptions end, some of society's stiffest challenges may have only just begun. This year marks the 40th anniversary of the renowned eruption of Mount St. Helens on 18 May 1980. Its observation accentuates awareness that science and society still confront the costly and potentially lethal sediment and hydrologic hazards that linger from cataclysmic events that transpired within minutes on a sunny Sunday morning. Two key hydrologic and geomorphic (hydrogeomorphic) issues, the lingering hazard posed by a volcanically dammed lake and relentless sediment delivery to distant communities, remain as problematic legacies of the eruption.

The eruption of Mount St. Helens (see the figure) was a pivotal event for understanding volcanoes and how an eruption affects the environment. It revealed that single eruptions can involve complex cascades of volcanic events that are physically intertwined and brought the recognition that a volcano can collapse abruptly in a gigantic landslide. Such a landslide can rapidly depressurize magma and generate a hot, high-velocity, debris-and-gas–laden cloud (a pyroclastic density current, or PDC) capable of sweeping and devastating hundreds of square kilometers of rugged landscape within minutes. The eruption delivered ruin to distant communities in the form of voluminous mudflows spawned by both swift scour and melting of snow and ice by the PDC and dewatering of the massive landslide deposit (1, 2). The physical and psychological impacts on people living in areas cloaked in the rain of volcanic ash carried downwind, known as tephra fall, required the health care system to confront many questions (3).

Factors that determined whether organisms survived the events involved not only the nature of the volcanic impacts, but also the season and time of day of the eruption. Notably, remnants of the pre-eruption biota—biological legacies—that persisted even in what appeared to be a lifeless landscape critically affected ecological recovery (4, 5). Despite the obvious and immediate consequences of the direct impacts of the eruption, the hydrogeomorphic responses to its events have left some of the most enduring and costliest legacies.

The colossal landslide that initiated the eruption, and subsequent PDC deposits that mantled its surface, buried 60 km2 of upper North Fork Toutle River valley. The landslide impounded new lakes and transfigured the basin of the iconic 270 million m3 Spirit Lake at the foot of the volcano. The lake's bed and surface were raised some 60 m and its outlet plugged (6). Consequent wet seasons showed deposits in this landscape to be exceptionally erodible. Floods, breakouts of impounded small lakes, and additional snowmelt-induced volcanic mudflows by later eruptions carved and enlarged new channels across fresh valley fill and delivered extraordinary sediment volumes downstream (7). This sediment clogged shipping lanes in the Columbia River and increased flood hazard in vulnerable communities by raising riverbeds (8). To mitigate flood hazards and restore commercial shipping, the U.S. Army Corps of Engineers (USACE) first embarked on a program of channel dredging (8), but swiftly concluded that it was neither fiscally nor physically sustainable. They subsequently constructed a 56-m-tall, 800-m-long sediment-retention structure (SRS) to stem the barrage of sediment (9).

The Spirit Lake level rose in the meanwhile, being effectively in a drainless bathtub, threatening to breach its blockage and unleash a massive flood and associated mudflow on downstream communities still reeling from impacts of the eruption. To mitigate impending disaster, USACE from 1982 to 1985 pumped water from the lake over the blockage while they bored a 2.6-km-long outlet tunnel through bedrock to lower and stabilize lake level and bypass the blockage (10). This induced additional channel erosion and downstream sediment delivery.

For many years the tunnel and SRS proved effective, but they no longer fully function. Thus, flood and sediment hazards related to the 1980 eruption must again be confronted (9, 11). The tunnel passes through several zones of sheared, weak rock. Deformation of those shear zones has episodically compromised tunnel integrity (12, 13). Consequently, costly (>$US 5 million) repairs have ensued, resulting in prolonged closures of the tunnel (12). With each prolonged closure, the lake has risen to precarious levels, approaching one that could potentially lead to breaching of the lake blockage (13). Tunnel repair costs, requisite closures, and implications for lake security have prompted the U.S. Forest Service (the tunnel owner) and USACE to examine alternative outlets (13). By late 1997, a decade after SRS completion, its impounded sediment had filled to spillway level, greatly reducing trapping efficiency and allowing sediment to bypass the structure (9). By 2007, USACE again dredged downstream river channels, but this time under more stringent and challenging environmental and permitting conditions. Consequently, they grapple with determining the most cost- and environmentally effective way to mitigate sediment-induced flood hazard under predictions that abnormal sediment delivery may continue for decades to come (9, 14).

The importance and societal impacts of massive sediment flushes following eruptions have crystallized after the 1980 Mount St. Helens and other recent eruptions (e.g., Pinatubo, Unzen, Chaitén, Merapi). Volcanically disturbed watersheds have generated some of the world's greatest sediment yields (15). Mount St. Helens shows that posteruption sediment redistribution clearly is one of the greatest and costliest challenges that societies must confront in volcanic regions. Furthermore, this societal challenge can linger for years, decades, or possibly centuries. In confronting the immediate need to protect societal assets from volcanically induced hydrogeomorphic hazards, the tensions between mitigating fluvial hazards in precariously built environments versus letting rivers have space to be rivers come into crisp focus. Four decades after the momentous 1980 eruption, geophysical and human consequences in this iconic volcanic landscape still challenge science and society.

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