Volcanic Studies at the U.S. Geological Survey's
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Mount St. Helens volcano in southwest Washington transformed one of the most
scenic alpine landscapes of the Cascade Range into a gray, barren wasteland in
only a few minutes. A catastrophic landslide and explosive eruption on May 18,
1980, devastated 550 square kilometers of forest, sent damaging mudflows down
rivers draining the volcano, and produced ash fallout hundreds of kilometers
to the east. Four years after the volcano's reawakening, Mount St. Helens
remains active, and the devastated landscape wrought by the May 18 eruption
continues to pose flood hazards to communities downstream from the volcano.
Sixteen subsequent eruptions have taken place in the horseshoe-shaped crater formed by the May 18 eruption. No large explosive event has occurred since October 1980. The last 11 eruptions have built a lava dome to a height of 240 meters by the slow intrusion of magma into its interior and the extrusion of lava onto its surface; the most recent eruption of Marcy 29, 1984, added a new stubby lava flow to the top of the dome.
U.S. Geological Survey scientists at the David A. Johnston Cascades Volcano
Observatory (CVO) in Vancouver, Washington, are studying the intermittent
eruptive activity of Mount St. helens and hazards posed by the effects of the
May 18 eruption. Many different monitoring techniques and instruments are used
to record the "daily pulse" of the volcano to evaluate the change of future
eruptive activity and to investigate fundamental volcanic processes. Studies
also are made of the river basins and the new lakes around the volcano and of
the massive debris avalanche that slid off the mountain on May 18, 1980. These
studies provide data to asses water-related hazards to downstream communities
from increased sedimentation, erosion, and flooding along the Toutle and Cowlitz
Rivers. Such hydrologic hazards likely will persist long after the decline of
eruptive activity at Mount St. Helens.
CVO is supported by the Geological Survey's Volcano Hazards Program, which is responsible for providing warnings of volcanic eruptions and related hazards in the United States. Information and advice are given to Federal, State, and local officials concerning specific volcanic hazards and the implementation of emergency-response plans. Activities designed to inform people of volcanic hazards include research on volcanic processes, monitoring active and potentially hazardous volcanoes, and mapping the types and the extent of volcanic deposits of past eruptions. The goal of the monitoring and research activities of CVO is to give timely warnings of eruptive and related hydrologic hazards at Mount St. Helens. Three types of written public statements about volcanic activity are issued by CVO to provide hazard information to the public and to governmental agencies:
Water-related hazard information is provided to the National Weather Service and
other Federal, State, and local agencies involved in flood mitigation planning.
Eruptive Activity of Mount St. Helens: March 1980 - March 1984The present activity of Mount St. Helens began in March 1980. A magnitude 4.0 earthquake on March 20 was followed by 2 months of intense seismicity and phreatic "steam-blast" eruptions. These events accompanied the intrusion of viscous magma into the volcano, shoving the north flank outward more than 100 meters and creating the famous "bulge".
On May 18, a magnitude 5.1 earthquake shook loose the steepened north flank,
resulting in the largest known landslide in historic time. Removal of 2.3 cubic
kilometers of material released pressure on the hydrothermal and magmatic system
within the volcano and triggered a devastating lateral blast to the north.
Within minutes, an ash-laden eruptive column rose more than 20 kilometers above
the volcano. Melting snow and ice formed mudflows and floods that raced down
almost all the valleys draining the volcano. The largest and most destructive
mudflow originated from water-saturated parts of the debris avalanche in the
North Fork Toutle River Valley.
Mount St. Helens erupted explosively five more times during 1980. None of these eruptions were as large as the events on May 18, but each eruption produced ash columns that rose 8 to 14 kilometers above sea level and hot, dry pyroclastic flows of pumice and ash that swept down the north flank as fast as 100 kilometers per hour. These pyroclastic flows deposited ash and pumice fragments in fanlike patterns of sheets, tongues, and lobes in an area extending up to 8 kilometers north of the vent. Individual pyroclastic flow units were generally less than 5 meters thick, and maximum temperatures recorded several hours after their deposition ranged from about 300 to 730 degrees C. The thickness of airfall deposits ranged from one-third to one-fortieth that of the May 18 airfall deposit at a given distance from the volcano.
Lava extruded from the vent and formed lava domes within a few days after the June 12, August 7, and mid-October explosive eruptions. The June and August domes were blown away by subsequent explosive eruptions, but the October dome survived to form the core of the still-growing dome. Domes are formed by thick, pasty masses of lava too sticky to flow very far from the vent. Lava of the Mount St. helens dome is dacite that contains a higher percentage of silica than the Hawaiian basalts and is about 1 million times more viscous.
The 11 eruptions since October 1980 have been dominantly nonexplosive events that have built a composite lava dome 750 x 675 x 240 meters high in the crater. Each eruption added a stubby lava flow to the dome. The lava flows commonly are extruded near the top of the dome, and over a period of several days, creep 1 to 5 meters per hour down one side; usually a few million cubic meters of new lava is added to the dome during one of these episodes. The dome-building eruptions in 1981 and 1982 were episodic and occurred every 1 to 5 months. Beginning in February 1982, the dome grew continuously by intrusion of magma into it and the extrusion of lava onto its surface; the most recent lava extrusion occurred in late March 1984. At the current rate of dome growth, about 1 million cubic meters per month, it would take 150 to 200 years to build Mount St. Helens to its former height, but it is unlikely that such a scenario will occur.
Small explosions sometimes precede or accompany the dome-building eruptions at Mount St. Helens; if they occur when snow mantles the crater floor, then they can produce mudflows (lahars) and snow avalanches. The explosive onset of the March 19, 1982, eruption hurled hot pumice and dome rocks against the 640-meter-high south crater wall, dislodging snow and rock that avalanched through the crater and down the north flank of the volcano. Deep snow in the crater melted quickly from the volcanic heat, forming a small lake from which a destructive flood swept down the north flank and into the North Fork Toutle River. About a day later, new lava erupted on the southeast flank of the dome.
In addition to the dome-building eruptions, vigorous emissions of gas and tephra have occurred from fractures and small craters on top of the dome since late 1980. These periodic outbursts usually last several minutes, occasionally sending ash plumes as high as 5 to 6 kilometers above the volcano. Most of the tephra consists of fragmented pieces of dome rock, not new liquid magma, in contrast to the more hazardous magmatic explosions of 1980.
The eruptive activity of Mount St. Helens presents unique opportunities for geologists to observe the formation of different types of volcanic deposits. Geologists typically study rocks and deposits that are thousands to millions of years old. By observing eruptive activity that occurs in the present, geologists are better able to recognize deposits preserved in the historic record. For example, a large hummocky deposit at the base of Mount Shasta Volcano in northern California has characteristics similar to those of the 1980 debris avalanche deposit at Mount St. Helens. Based on detailed studies of the Mount St. Helens debris avalanche, geologists now can infer with greater confidence that a large landslide also produced the deposit near Mount Shasta. Stratigraphic studies at Mount St. Helens include studies of the debris avalanche, the lateral blast, mudflows, pumiceous pyroclastic flows, airfall tephra, and lava domes. Monitoring Activities at Mount St. Helens and Other Cascade VolcanoesThe continuing eruptions of Mount St. Helens provide an unusual opportunity for scientists to study volcanic activity and to devise and test methods for predicting eruptions. Many successful predictions have been issued for eruptions since June 1980. Eruption prediction and information about volcanic activity at Mount St. Helens provide the basis for hazard warnings of eruptive activity to the public and to local governments.Volcano monitoring involves a variety of measurements and observations designed to detect changes at the surface of a volcano that reflect increasing pressure and stresses caused by the movement of magma, or molten rock, within or beneath it. An eruption occurs when magma rises from its source or from a storage reservoir and finally reaches the Earth's surface. As it rises, the magma fractures overlying rocks, which causes earthquakes, and parts of the volcano deform as magma approaching the surface makes room for itself. Monitoring at Mount St. Helens chiefly involves the measurement of surface deformation, the investigation of earthquakes generated beneath the volcano, and the study of changes in gas emission rates accompanying the underground movement of magma. Additional geophysical and geochemical information is gathered through sampling of newly erupted lava and tephra, studies of thermal patterns on the dome, surveys of local electrical and magnetic fields, measurements of changes in the Earth's gravity field, examination of photographs, and measurements of temperature at fumaroles.
Many of the methods used to monitor Mount St. Helens were developed at the U.S.
Geological Survey's Hawaiian Volcano Observatory where the activity of Kilauea
and Mauna Loa shield volcanoes is monitored. Although the techniques are
similar, their application and interpretation have been modified and adapted to
Mount St. Helens and other stratovolcanoes of the Cascade Range.
Seismic StudiesSeismic signals from 5 seismometers on the flanks of Mount St. Helens, one in the crater, and 10 others within 40 kilometers of the volcano are radioed to the Geophysics Laboratory at the University of Washington. Signals from several of these stations are also radioed to and recorded at the Cascades Volcano Observatory. This detailed seismic network enables seismologists to distinguish between different types of volcanic earthquakes and surface events. Although the seismic precursors to the May 18, 1980, eruption did not specify the time of its onset, seismologists have learned to recognize certain characteristic patterns of seismic activity that precede and accompany the subsequent eruptions. By plotting the cumulative seismic strain energy release of various types of seismic disturbances versus time, eruptions have been predicted from a few hours to several days in advance.
Earthquake data from the Mount St. Helens seismic network are stored on computer files at the University of Washington in Seattle. Seismologists review and classify the seismograms, or "seismic signatures," from several local stations each day; during periods of high earthquake activity, seismologists monitor the records 24 hours a day. The following major types of seismograms have been recognized at Mount St. Helens: (1) deep earthquakes and those located away from the volcano, which produce high-frequency signatures and sharp arrivals similar to tectonic earthquakes, (2) shallow earthquakes, located under the dome at depths of less than 3 kilometers, which produce medium-to-low-frequency seismic arrivals, (3) surface events, such as gas and tephra events, rockfalls associated with dome growth, and snow and rock avalanches from the crater walls, which produce complicated signatures with no clear beginning or end, and (4) harmonic tremor, which is a long-lasting, very rhythmic signal whose origin is not well understood but which is often associated with active volcanoes.
The rate of activity of the various categories of seismic events is used to
assist in predicting volcanic activity at Mount St. Helens. An increasing
number of shallow volcanic earthquakes were observed several days to 2 weeks
before each dome-building eruption from 1980 through 1982.
As the number of earthquakes increase,
total seismic energy release is calculated and plotted against time.
The observation of a sudden upward turn in this smoothly accelerating curve a
few hours before the eruption begins is the basis for relatively short-term
predictions. Once the eruption is underway, shallow volcanic earthquakes cease,
and surface events from rockfalls dominate the records.
Increased seismic activity also preceded the post-May 18 explosive eruptions of 1980. Each of the seismic precursors were of a slightly different character, but two categories were recognized: shallow volcanic earthquake precursors and harmonic tremor precursors. Harmonic tremor is a nearly continuous train of vibrations lasting from a few minutes to several hours. An increase in shallow earthquakes preceded the July and October 1980 eruptions when a lava dome was present in the crater. Harmonic tremor preceded the May 25, June 12, and August 7, 1980 eruptions when the vent was open and no dome existed. Having learned from the seismic buildup preceding the May 25, explosive eruption, seismologists were able to issue warnings of the subsequent explosive events by at least 2 hours. Although a characteristic pattern of seismic activity has accompanied most eruptions, there have been departures from the pattern. The explosive eruptions of 1980 were followed by swarms of deep earthquakes (deeper than 5 kilometers). In contrast, none of the subsequent dome-building eruptions were followed by deep earthquakes that could be related to the eruptions. The March 19, 1982, dome-building eruption began with an explosion and was preceded rather than followed by both deep and shallow earthquakes. The continuous dome-growth eruption that began in early February 1983 was preceded by an unusually small increase in shallow earthquake activity. Earthquakes and surface events occurred daily reflecting the continuous dome-growth activity in 1983. Deformation StudiesShortly after the May 18, 1980, eruption, geologists reestablished a surveying network on the volcano to measure changes that might signal additional eruptive activity. Glass prisms atop heavy steel towers on the volcano's upper flanks were surveyed with an electronic distance meter (EDM) and theodolite from instrument towers placed at the base of the volcano. Frequent measurements showed small movements during the summer of 1980, some of which may have indicated swelling before the explosive eruptions. No Systematic deformation of the outer flanks of the volcano has been detected since 1980.
The largest changes measured in 1980 were inside the new crater near the eruptive vent. As a consequence of these observations, monitoring has been focused inside the crater since fall 1980. Geologists visit the crater several times each week to measure deformation of the crater floor and the lava dome before eruptions. At the end of each field day, total measured displacements of ground cracks, thrust faults, and the dome are plotted against time. The observation of a gradual and then a rapid change in the curves a few days to several weeks before an eruption begins is the basis for relatively long-term predictions. Ground-Crack MeasurementsNew ground cracks appeared on the crater floor from several days to 2 to 4 weeks before all the 1980 to 1982 eruptions. The cracks, commonly tens of meters long and tens of centimeters wide, extended outwards from the dome like spokes from the hub of a wheel. Incandescent rock was visible in some cracks, and temperatures of escaping gas were measured as high as 840 degrees C. Measured with a steel tape, the cracks commonly showed continual widening that accelerated before eruptions. Such accelerated movement was used to predict several eruptions in 1981 and 1982.
Thrust-Fault MeasurementsFrom 1980 to 1982, parts of the crater floor became slightly wrinkled several weeks before eruptions. A few wrinkles developed into thrust faults, a low-angle fracture, along which rocks above the fracture are pushed over rocks below the fractures. By summer 1981, a complex system of thrust blocks had disrupted much of the southwestern part of the crater floor. The thrust faults formed as rising magma forcefully ruptured the crater floor, shoving parts of it upward and outward from the vent toward the rigid crater walls. Before the August 18, 1982, eruption, the leading edge of one thrust fault grew from less than 30 centimeters to roughly 5 meters high a few days before the eruption.Movement along the faults was monitored by repeated measurements between benchmarks on either side of the fault by using a steel tape and by leveling between points on the upper and lower plates. The rate of movement accelerated before eruptions, and this provided the most consistent and reliable relatively long-term (1-3 weeks) predictive tool at Mount St. helens during 1981 and 1982. Rockfalls from the dome and crater walls have buried some major thrust faults under rock debris, others have been overridden by the dome itself; but new thrust faults occasionally appear, as in October 1983. Tilt MeasurementsIn addition to cracking and faulting, the crater floor tilts before eruptions. Electronic tiltmeters, which are widely used in Hawaii, measure changes in slope or inclination of the ground surface. Tiltmeters specifically designed for use at Mount St. Helens have been installed on the crater floor tens to several hundred meters from the dome. Tilt data contributed to accurate predictions for eruptions during 1981 and 1982.The tiltmeters, which employ two sensitive bubbles mounted at right angles on a 15-centimeter base plate, measure tilt of the crater floor in two directions. The direction of tilt is generally outward from the dome but is sometimes complicated by nearby cracks or faults. Amount of tilt is expressed in microradians, which is the angle turned by a 1-kilometer-long rod if one end is raised 1-millimeter. Although these tiltmeters are capable of measuring one-tenth of a microradian, precision is limited to 5 to 10 microradians in the crater because of surface thermal effects. Data from the crater tiltmeters are radioed directly to CVO in Vancouver and are stored in computer files. Tiltmeters provide the only realtime information about crater deformation and are, therefore, especially valuable when field work is impossible because of poor weather or hazardous volcanic activity.
Tilting of the crater floor began several weeks before each eruption in 1981 and
1982, accelerated a few days before, and, on several occasions, abruptly
reversed direction minutes or hours before the eruption began; for example, tilt
before the March 19, 1982, eruption at one station increased from about 14
microradians per day 3 weeks before the event to 360 microradians per day on
March 19. The tilt reversed direction about 30 minutes before the eruption
began.
Lava-Dome MeasurementsMeasurements show that the dome expands as magma moves up into it before eruptions. Repeated surveys using an electronic distance meter (EDM) and theodolite between points on the crater floor and targets placed on the dome reveal movements that speed up as the eruption nears. A target on the west side of the dome was moving roughly 2 centimeters per day 2 weeks before the May 14, 1982, eruption; these movements increased to about 200 centimeters per day by May 13. Such accelerations were frequently used to predict eruptions in 1982.
Generally, all sides of the dome were monitored in 1981 and 1982 from four to
five stations on the crater floor. Experience has shown that the dome deforms
differently at each of the stations, thus making it necessary to monitor more
than one side for reliable predictions. During the winter months, one and
sometimes two sides of the dome can be monitored; it is no longer possible
to monitor the east sector of the dome because of hazardous rockfalls
from the dome and the east crater wall. This technique has become essential fro
predicting the most recent eruptions because most of the ground cracks and
thrust faults have been buried by the dome and associated rockfall debris. This
type of measurement also documents movements of the dome during prolonged
eruptions, such as that of 1983, and is used to foretell significant changes in
such continuous activity.
Gas Emission StudiesGas emissions are measured regularly in conjunction with seismicity and ground deformation to monitor eruptive activity. Mount St. Helens continuously emits volcanic gas from fumaroles on and around the dome. Most of the gas emitted by the volcano is water vapor, but emissions also include sulfur dioxide, carbon dioxide, hydrogen, and lesser amounts of helium, carbon monoxide, hydrogen sulfide, and hydrogen chloride.
Gas studies include (1) frequent airborne measurements of sulfur dioxide and,
in 1980 and 1981, carbon dioxide in the plume and (2) less frequent sampling of
gases from crater fumaroles.
The emissions of sulfur dioxide are measured in the plume by a
correlation spectrometer (COSPEC) designed originally
for pollution studies. The instrument
measures the amount of solar ultraviolet light absorbed by sulfur dioxide
in the plume and compares it with an internal standard.
Three to six traverses are made beneath the plume at right angles to
the plume trajectory several times
each week to calculate daily emission rates.
The emission rates of sulfur dioxide peaked during summer 1980 at about 1,500 tons per day, decreased rapidly in late 1980, and remained low at about 100 tons per day through 1983. Emission rates of carbon dioxide decreased rapidly in late 1980 until they were below the detection limit of 1,000 tons per day. These patterns correspond generally to a change in eruptive style from the explosive activity of 1980 to the now predominantly nonexplosive activity. The patterns suggest steady outgassing of a single batch of magma under the volcano to which no significant new magma has been added since mid-1980. Increased rates of sulfur dioxide emissions measured before several nonexplosive eruptions are interpreted as the result of accelerated degassing of a small volume of magma as it moved toward the surface. During the nonexplosive eruptions, gas emissions remained elevated during the active extrusion of lava and generally dropped to preeruption levels once extrusion stopped. The occasional outbursts of gas and tephra are accompanied by brief, sudden increases in the emission rate of sulfur dioxide, water vapor, and probably other gases as well. It is not known whether this increase in gas is derived directly from magma within the dome or released during periodic, geyserlike flashing of a shallow hydrothermal system. Dome CompositionSince October 1980, each eruption has added a new flow of viscous dacitic lava to the dome. Chemical and mineralogic analyses of dome samples show that lava composition throughout successive eruptions has remained nearly constant. This suggests that the magma feeding the eruptions has not undergone any significant changes and is consistent with the model of a single, shallow magma reservoir supplying the eruptions. Whether the chemical composition of future extrusions remain the same or show systematic changes, new analyses must be interpreted in the light of all other observations to help anticipate the character of future behavior.Thermal ObservationsNighttime aerial observations of the lava dome also have been useful in monitoring volcanic activity. An image intensifier attached to a 35-millimeter or video camera can detect hot spots on the dome that are not visible to the naked eye. The intensifier magnifies the glowing spots 20,000 to 40,000 times. Increased visible glow is sometimes observed a few days before and eruption. This technique, currently under development, shows promise for monitoring the dome should work in the crater be curtailed for any reason.Other VolcanoesSurvey networks and tilt stations have been established on other volcanoes of the Cascade Range: Mount Baker and Mount Rainier in Washington, Mount Hood and Crater Lake in Oregon, and Mount Shasta and Lassen Peak in northern California. Current plans are to remeasure these networks every 3 years, unless conditions, such as increased seismicity, warrant more frequent monitoring. Earthquake activity is continuously monitored at these volcanoes. Monitoring earthquakes and ground movement on these volcanoes will probably record changes before future eruptive activity, but it is unlikely that the monitoring will define the exact time or type of an eruption as precisely as is currently possible at Mount St. Helens.The drama of the May 18, 1980, eruption of Mount St. Helens captured the attention of people throughout the world to a degree matched by few natural events in recent decades. However, the energy and volume of this eruption were exceeded considerably by several eruptions elsewhere in the world during the past century. Even so, the fact that a great natural disaster occurred in one of the highly developed countries has brought volcanoes in general and Mount St. Helens in particular a degree of recognition and visibility not previously achieved. Systematic observations and monitoring of the activity since its beginning in 1980 probably have exceeded those associated with any major eruption elsewhere, thus providing both an opportunity for education on a worldwide scale and a responsibility for disseminating the lessons learned.
Scientists and public officials from at least 25 countries have visited CVO and
Mount St. Helens and have gained improved insights into volcanic processes and
into how society interacts with natural disasters. In turn, scientists of the
U.S. Geological Survey's Volcano Hazards Program have participated in monitoring
active volcanoes in other countries, including Indonesia, Papua New Guinea, and
New Zealand, and have visited areas of active volcanism in several other
countries and have consulted with their scientists. This kind of interaction is
of immeasurable value in helping the international community of volcanologists
to deal with the vital goals of improving the ability to predict volcanic
eruptions and to mitigate volcanic risks.
The Changing Hydrology at Mount St. HelensThe effects of the May 18, 1980, eruption of Mount St. Helens significantly increased the threat of flooding along rivers draining the volcano, especially the Toutle and lower Cowlitz Rivers. The erosion of unconsolidated volcanic debris around the volcano and the possible failure of debris dams impounding Spirit, Coldwater, and Castle Lakes pose hazards to communities downstream from the volcano. Steps have been taken by the U.S. Army Corps of Engineers to reduce the threat of flooding, and discussion is currently underway to implement long-term solutions. Data collected by U.S.Geological Survey hydrologists from the Cascades Volcano Observatory and from Tacoma, Washington, are given to Federal, State, and local officials who are responsible for flood-mitigation and emergency-response plans.Hydrologic MonitoringThe May 18, 1980, eruption significantly increased the rate of surface runoff during storms and the availability of readily erodible sediment by destroying vegetation and by depositing loose debris over a wide area north of the volcano. The directed lateral blast stripped trees from most hillslopes within 11 kilometers north of the volcano and leveled nearly all vegetation as far as 20 kilometers in a 180-degree arc north of the mountain. The blast deposited blocks, smaller rock fragments, and organic debris over the 550-square-kilometer area in layers to more than 1 meter in thickness. Surrounding this zone of toppled vegetation is a narrow 100-square-kilometer band of scorched, but standing, timber in which sandy deposits are as thick as 8 centimeters.
Rivers with headwaters in the blast area have a rapid streamflow response to rainfall, owing to reduced infiltration rates on hillslopes and low roughness along channels. Streams now respond more quickly to a given amount of rainfall and produce higher peak flows as rainfall is quickly flushed through the drainage system. Greater streamflow increases the erosion and transportation of sediment from hillslopes and river channels; deposition of this debris in the lower reaches of the Toutle and Cowlitz Rivers reduces channel depths, thereby increasing the possibility of flooding. Flood levees, channel dredging, and debris-retention structures built by the U.S. Army Corps of Engineers have thus far prevented serious flooding to communities along the Toutle and Cowlitz Rivers. Lahars, or mudflows, and floods accompanying the May 18 eruption rushed down nearly all the streams draining the volcano and spread over valley floors, raised channel beds, and destroyed roads, bridges, and homes. Lahars formed during the initial blast, and pyroclastic flows occurred in the Smith, Pine, and Muddy River drainages on the east flank of the cone and in the South Fork Toutle River on the west flank. The most voluminous and destructive lahar originated by the slumping and flowing of water-saturated parts of the debris-avalanche deposit. This lahar peaked near the mouth of the Toutle River at midnight and left deposits 1 meter thick on parts of the flood plain and 5 meters thick in the channel. These deposits immediately reduced channel depths and increased the possibility of flooding along the Toutle and lower Cowlitz Rivers. Smaller lahars were formed by the melting of debris-laden ice and snow during the afternoon of May 18.
The debris avalanche that triggered the eruption slid north into Spirit Lake
and west 25 kilometers down the North Fork Toutle River valley, covering the
valley floor with unconsolidated debris to an average depth of 45 meters and
as much as 180 meters in some places. Rapid erosion resulting from the
breaching of numerous ponds and lakes on the deposit and surface runoff have
produced a new drainage system on the avalanche. Streams following the
initial drainage pattern quickly eroded narrow channels because of the
generally steep slopes and the readily erodible character of the avalanche
deposit. Channels more than 300 meters wide and 45 meters deep have been
carved by the new North Fork Toutle River. Nearly 4 years after the
devastating eruption, erosion rates remain high, and the channels display
complex, alternating scour-and-fill sequences.
The debris avalanche raised the level of Spirit Lake 64 meters and dammed its natural outlet even higher. Many small ponds filled closed depressions on top of the avalanche deposit, and several lakes formed in tributaries dammed by the avalanche; the largest lakes formed in the tributaries of Coldwater and Castle Creeks. In late 1980, some of the ponds overtopped and swiftly eroded their new outlets. The rapid release of water generated highly erosive flows on the avalanche and transported large volumes of sediment down the lower reaches of the Toutle and Cowlitz Rivers.
Failure of the debris dams holding
Spirit, Coldwater, and Castle
Lakes would result in catastrophic mudflows comparable to or larger than those
of May 18, 1980. Controlled outflow channels have been constructed to stabilize
the water levels of Coldwater and Castle Lakes, and water from Spirit Lake is
currently being pumped into the Toutle River by the Corps of Engineers as a
temporary measure to control its level. Permanent solutions are being
considered to alleviate the flood threat from Spirit Lake.
The Geological Survey gathers information on hydrologic hazards at Mount St. Helens in the following ways: (1) by measuring the annual rates of erosion, transportation, and deposition of sediment along streams draining the volcano, (2) by monitoring the water elevations and the stability of the debris dams impounding Spirit, Coldwater, and Castle Lakes, and (3) through research on the flow characteristics of lahars and streams with high sediment loads. Erosion, Transportation, and Deposition of SedimentThirteen gaging stations were constructed after the May 18 eruption to measure water and sediment discharge of the rivers draining Mount St. Helens; these stations supplement those already in place on reservoirs and rivers around the volcano. Gaging stations continuously record the water-surface elevation or stage of a river. Stream discharge is calculated from the relationship between this recorded stage and periodic manual discharge measurements. Hydrologists also collect water samples at the gage sites and analyze them to determine total suspended sediment transported by the streams.
The network of river gages provide information for flood forecasting and for
long-term sediment-transport trends. These data are used by the National
Weather Service to warn of severe flooding conditions and by the Corps of
Engineers to develop sediment-control solutions.
Since May 18, 1980, sediment transport rates for the rivers flanking Mount St. Helens, especially the Toutle River, have been among the highest in the world. More than 20 million tons of suspended sediment was transported from the Toutle River basin in the first 7 months after the May 18, eruption, or 15 million tons in only 13 days. About 39 million tons of suspended sediment was transported from October 1981 to September 1982, enough to cover an average city block to a depth of 8 kilometers. Since 1980, storms have been of only low to moderate intensity; consequently, less than 5 percent of the total volume of the avalanche deposit has been removed by erosion, so it will persist as a sediment-management problem for many years. More than 150 cross-sections of river channels are surveyed regularly to determine areas of erosion and deposition along rivers draining Mount St. Helens. These repetitive surveys measure bank and channel erosion and channel deposition at specific locations. Repeated aerial photographs also are used to identify sediment sources and sinks.
In many places since the 1980 eruptions, channel modifications have been equal
to or greater than those resulting directly from the damaging lahars
on May 18. Generally, erosion and sediment transport by channel widening and
downcutting dominate the upper reaches of the drainage basins, and aggradation
and sediment transport dominate the lower reaches.
Lake MonitoringSix lake gages, maintained by the Geological Survey in cooperation with the National Weather Service and the Federal Emergency Management Agency, monitor the water levels of Spirit, Coldwater, and Castle Lakes. The gages serve as an emergency warning system if one of the debris dams fails. Each gage has at least two recording instruments that transmit several lake elevations each hour by way of a satellite to a ground receiving station in Tacoma, Washington. If a lake level drops faster than the specified rate, alert transmissions send lake elevations every 5 minutes.
Overtopping of the debris dams due to filling from normal precipitation was considered to be the most likely cause of lake breakouts and resulting floods; controlled outlet channels and the Spirit Lake pumping operation have eliminated this possibility. However, a sudden influx of a large volume of volcanic debris from an eruption of Mount St. Helens could raise rapidly the level of Spirit Lake. An eruption producing pyroclastic flows more voluminous than those of May 18, 1980, would be necessary to cause overtopping. Several geologic and geophysical studies evaluate and monitor the potential instability of unconsolidated material that blocks the lakes. Failure of these debris dams could result from slumping of the dams, liquefaction from shaking during earthquakes, or headward erosion of gullies and channels. The studies suggest, however, that these possibilities are unlikely in the near future. A seismic zone about 1,000 kilometers long trends north-northwest through Mount St. Helens and beneath the debris-avalanche deposit. During recent decades, several significant earthquakes have occurred along this zone, the largest of which was magnitude 5.5 and occurred in February 1981. Ground-water wells and seismometers on the surface of the avalanche deposit and in holes 6 to 30 meters deep are used to monitor the response of the unconsolidated debris to earthquake activity. The relatively narrow debris blockage at Castle Lake is most subject to slumping or gravitational failure. Instruments in drill holes as deep as 30 meters monitor slope movements of the Castle and Spirit Lake dams, and ground-water tables are recorded at all three lake blockages. Erosion is monitored by repeated photographs and channel geometry surveys. ResearchThe Hydrologic Monitoring Program provides hazard information and improves understanding of the hydrologic processes involved in the devastation and recovery of areas affected by the May 18, 1980 eruption and lahars. Information collected by the monitoring techniques are used to investigate factors affecting the stability of stream channels and the fluid dynamics of flows that transport high sediment loads.The May 18 lahar deposits and small debris flows that continue to occur on the volcano also help hydrologists to interpret deposits in the historic record at Mount St. Helens and other Cascade volcanoes. Newly recognized pre-1980 lahar deposits in the Toutle River valley are interpreted to have been emplaced from previous breakouts of Spirit Lake. Lahar-hazards studies on Mount Hood Volcano in northern Oregon also have been aided greatly by the study of recent deposits at Mount St. Helens.
Mount St. Helens offers an unusual opportunity to study the flow characteristics
of lahars and debris flows and to develop models for predicting their behavior
and effects downstream. Information from these studies and the development of
better scientific techniques will continue to improve understanding and the
expertise needed to manage water resource problems caused by future volcanic
eruptions.
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