Thirty-seven years after the May 18, 1980, eruption of Mount St. Helens, scientists, engineers, land managers, and Federal, State, and County officials are still grappling with a challenge created by the eruption—how to prevent potentially massive downstream flooding by a sudden release of water from Spirit Lake.
A new report from the National Academies of Sciences, Engineering, and Medicine presents a framework to guide federal, tribal, state and local agencies, community groups, and other interested and affected parties in making long-term decisions about the management of the Spirit Lake and Toutle River system. It suggests the process include broad participation by groups and parties whose safety, livelihoods, and quality of life will be affected by decisions about the lake and river system, and that decisions be supported by a quantitative risk assessment, benefit-cost analyses and analyses of other data.
The May 18, 1980 eruption began with an enormous landslide that slammed into Spirit Lake, blocking its natural outlet and raising the lake level by 197 feet. Without an outlet, the water level rose with each rainstorm and seasonal snowmelt, threatening to breach the blockage and produce a catastrophic flood for communities downstream.
To mitigate the hazard, in 1984-1985, the U.S. Army Corps of Engineers constructed an 8,500 foot long, 11 foot diameter tunnel through a bedrock ridge on the west side of Spirit Lake. The lake drains through the tunnel and into the North Fork Toutle River. The tunnel has successfully controlled the lake level since 1985.
Over time, the tunnel has required costly repairs and more are expected in the future. When sections of the tunnel are repaired or upgraded, it is closed for many months and the lake level rises. During each repair, lake levels have approached maximum "safe" levels. If an exceptional weather event, large eruption or major earthquake coincided with an extended tunnel closure that allowed lake levels to rise above 'safe' levels, the lake could potentially breach the blockage and send flood waters downstream. Communities along the Toutle, Cowlitz, and Columbia rivers could be impacted, leading to loss of life and damage of more than $1 billion.
The new report will help management agencies develop long-term solutions to meet the risk-management challenges. The report, A Decision Framework for Managing the Spirit Lake and Toutle River System at Mount St. Helens, is available from the National Academies Press online or by calling 1-800-624-6242.
More information, including a semi-quantitative risk assessment, is available in a publication prepared by U.S. Forest Service and USGS scientists, The geologic, geomorphic, and hydrologic context underlying options for long-term management of the Spirit Lake outlet near Mount St. Helens, Washington.
For over 40 years, scientists have used the latest techniques and technologies to track ground deformation (or surface changes) at Cascade Range volcanoes. The work evolved from sporadic and sparse reconnaissance surveys in the early 1970s to networks of continuously recording GPS stations, semi-permanent GPS stations deployed for weeks to months at a time, and space-based InSAR observations.
The networks, long-term data collection, research, analysis and modeling help shed light on how arc volcanoes work before, during and after eruptions. Here is a brief recap of what's up in the Cascades.
Uplift at South Sister
One volcano has persistent uplift—South Sister. South Sister became the focus of intensive geodetic work in 2001 when InSAR data revealed inflation centered about 6 km (4 mi) west of the summit. The uplift at South Sister is likely related to magma emplacement at a depth of 5-7 km (3-4 mi). The inflation started around 1997 at a maximum rate of 3-5 cm (1-2 in) per year. Since that time, the rate of uplift has declined due to a decrease in the magma accumulation rate, a relaxation of the magma body or a slow release of gases within the magma. Subtle inflation continues as of 2017 but at a low rate of about 5 mm (0.2 in) per year.
Persistent deflation at Mount Baker, Medicine Lake and Lassen volcanic center
Since perhaps as early as the late 1970s, Mount Baker has contracted at a rate of about 1-2 mm (0.04 to 0.08 in) per year. The deflation, which is centered under the volcano's north flank, could be due to the densification of a magma body emplaced in 1975 and/or establishment of a connection between a deep magma body and the surface that allows gases to escape.
Medicine Lake is subsiding at an average rate of about 8-9 mm (0.3 in) per year since at least 1954. The volcano occupies a tectonic setting along the western edge of the Basin and Range extensional province so subsidence is likely the result of crustal thinning due to tectonic extension and a slow sinking of the volcano's mass. Subsidence could also result from cooling and crystallizing of magma bodies at depth.
The Lassen volcanic center is also in an area of tectonic extension. Subsidence of at least 6 mm (0.2 in) per year has been going on since at least the 1980s. The cause of the subsidence is probably a combination of tectonic extension and contraction of the magmatic system or changes in hydrothermal processes.
Uplift and subsidence at Mount St. Helens
Deflation of Mount St. Helens' deep magma system was accompanied by uplift of the crater floor in September 2004, as magma rose to the surface and ultimately erupted. As the 2004-2008 eruption ended, the network around the volcano recorded a transition from deep deflation to subtle inflation due to the repressurization of the magma reservoir. Inflation decayed to background levels by early 2013 and as of 2017, deformation is at or below background levels.
No changes detected at Mount Rainier, Mount Hood, Newberry Volcano, Crater Lake and Mount Shasta
No significant volcano-related deformation has been detected at five volcanoes since the 1980s—Mount Rainier, Mount Hood, Newberry Volcano, Crater Lake and Mount Shasta. This sets an important baseline for future work and will be especially valuable in interpreting the sources of any future unrest.
Additional data needed to assess Glacier Peak, Mount Adams and Mount Jefferson
There are not sufficient data for the remaining three volcanoes for a rigorous assessment—Glacier Peak, Mount Adams and Mount Jefferson. All three volcanoes are located, to some extent, in remote areas, making field work a logistical challenge; a more rigorous analysis would yield a more definitive assessment.
Read more in the article Volcano geodesy in the Cascade arc, USA.
Mount Baker was the focus of a lidar (Light Detection and Ranging) survey that returned high-resolution digital topographic data. These data provide a digital map of the ground surface beneath forest cover, revealing landforms that record the glacial and volcanic history with astounding clarity. The DEM (Digital Elevation Model) dataset and a hillshade image of Mount Baker are now available online in the USGS Data Release: "High-resolution digital elevation dataset for Mount Baker and vicinity, Washington, based on lidar surveys of 2015." The DEM must be opened by software that can read and process GIS data, but the hillshade zip file includes a .tif that can be opened by an image viewer.
Read more and download the data at High-resolution digital elevation dataset for Mount Baker and vicinity, Washington, based on lidar surveys of 2015.
Researchers wrapped up a project at Castle Lake, near Mount St. Helens, mapping the lake bottom and refining measurements of lake volume and surface area. These data will be used in the numerical dam-break models that evaluate potential flood impact to communities downstream of Castle Lake, which include Kid Valley, Toutle, Castle Rock, Kelso, and Longview. While the 65-feet-tall mound of debris that blocked Castle Creek to form Castle Lake is tall enough to retain the lake, a concern remains about whether it is susceptible to other modes of internal failure during extreme hydrologic conditions or during a large earthquake.
The lake formed after South Fork Castle Creek was dammed by the debris avalanche from Mount St. Helens' May 18, 1980, eruption. Almost immediately after the 1980 eruption, the USGS, US Army Corps of Engineers and US Forest Service began evaluating the risk of Castle Lake catastrophically breaching the unconsolidated blockage. An emergency spillway was constructed in late 1980 to stabilize lake-surface elevation. The recent research allows for the computation of lake volume from near real-time lake elevation measurements from a monitoring station or from remotely sensed imagery, and to assess potential downstream hazards.
While most people are aware of the big eruption on May 18, 1980, the volcano continued to have smaller eruptions on May 25, June 12, July 22, August 7, and October 16-18, 1980, that produced pyroclastic flows and other depositional features. USGS geologists were on the scene immediately following the eruptions to map Earth's newest surfaces created by these deposits.
The paper maps they created in 1990 are now available in a modern digital format for use in volcano research. The data release consists of attributed vector features, data tables, and the cropped and georeferenced scans from which the features were digitized, in order to enable visualization and analysis of these data in GIS software.
To read more about these maps and download the GIS data, visit Database for geologic maps of pyroclastic-flow and related deposits of the 1980 eruptions of Mount St. Helens, Washington.
An earthquake swarm began near Mount Hood Monday night ~7 km (4.3 miles) south-southeast of the summit in the White River Valley, an area that commonly exhibits seismicity. It started October 9 at 19:51 UTC (12:51 PDT local) with the largest earthquake a M2.8 at 2:45 UTC on October 10 (1945 PDT on October 9). 14 earthquakes have been located by the Pacific Northwest Seismic Network (PNSN) through October 11, all with depths of 7-8km (4.3 - 5 miles) below sea level.
Such swarms are common in the vicinity of Mount Hood, with the last swarm occurring in May 2016 (largest event M 2.9). Summaries of past swarms near Mount Hood can be found on the PNSN website and on the Mount Hood monitoring webpage.