DESCRIPTION:
Mount St. Helens Hydrologic Hazards

Given the current, relatively quiet, eruptive behavior of Mount St. Helens, debris flows and floods at present constitute the greatest hazards related to volcanic activity. The potential for mudflows and floods was increased by the existence of new ponds and lakes formed when the debris avalanche of May 1980 blocked parts of the preexisting drainage to serve as natural dams. As these natural dams are composed of loose, easily erodible volcanic debris, they are structurally weak and could fail, which would trigger mudflows and floods.

A large volume of snow and ice is presently accumulating in the Mount St. Helens crater, protected by the shade of the high, steep crater walls. This accumulation provides a growing potential water source for lahars in the North Fork Toutle River valley. It is already mixed with rock debris eroded from the crater walls, and this debris would augment the formation of a lahar. It is possible that a large eruption could melt most or all of this snow and ice in a matter of tens of minutes. A very small eruption in 1982 rapidly melted enough snow and ice in the crater to trigger a 4 million cubic meters (5.2 million cubic yards) flood that transformed into a lahar and flowed all the way to the Cowlitz River. At the present time (1995), about 53 million cubic meters (70 million cubic yards) of snow and ice has accumulated. If completely melted, this would produce about 38 million cubic meters (50 million cubic yards) of water. At the present rate of accumulation, the volume of snow and ice will double in about 15 years.

Permanent and seasonal snow and ice also blanket the outer flanks of Mount St. Helens. A sufficient volume exists there in winter or spring to produce flank lahars similar in magnitude to those of May 18, 1980, if another large eruption were to occur. Lahars formed on the outer flanks can be expected to be substantially smaller than flows generated in the crater.

Mount St. Helens Lakes Menu

Mount St. Helens Snow and Ice Accumulation Menu

Streams that head on the volcano enter three main river systems -- the Toutle River on the north and north-west, the Kalama River on the west, and the Lewis River on the south and east. The streams are fed by abundant rain and snow that dump an average of about 140 inches of water on Mount St. Helens a year, according to National Weather Service data. The Lewis River is impounded by three dams for hydropower generation. The southern and eastern sides of the volcano drain into an upstream impoundment, the Swift Reservoir, which is directly south of the volcano.

Two major river basins in Washington State, the Cowlitz River and Lewis River basins, were affected by excessive sediment loads following the May 18, 1980, eruption of Mount St. Helens. The Cowlitz River drainage basin has an area of 2,480 square miles and includes the Toutle River basin, which was severely altered by the eruption and was inundated by mudflows. Prior to the eruption, the Toutle River was a typical Cascades Range stream, having a cobble bed, forested watershed, and headwaters at several glaciers. Devastation of the upper basin by the volcanic blast, the massive collapse of the volcano's north face into the North Fork Toutle River valley, and deposits from the resulting debris flows and mudflows provided enormous supplies of sediment for transport. Storm flows immediately eroded large volumes from the debris avalanche and mudflow deposits, and induced widespread collapse of unprotected bank material along the Toutle River. Gullying and channel extension on the North Fork Toutle River debris avalanche made additional volumes of sediment available for transport in subsequent years. Intensive and periodic sediment sampling began in the Toutle River basin immediately following the eruption.

Mount St. Helens River Drainages Menu

A number of natural and human-made lakes exist close to the volcano in the North Fork Toutle and Lewis River valleys. The uppermost lake in the Lewis River valley, Swift Reservoir, receives drainage from the volcano via Swift Creek, Pine Creek, and Muddy River. In 1980, lahars descending these streams dumped about 14 million cubic meters (18 million cubic yards) of sediment and water into the lake, abruptly raising the lake level 0.85 meters (2.8 feet). Because the operators of the reservoir, Pacific Power and Light, lowered the lake level about 18 meters (23 feet) below normal in anticipation of possible lahars, the small lake-level rise and the 0.4 meter (1.3 feet) accompanying wave posed no threat to the dam. It is assumed that (1) future lahars reaching Swift Reservoir would not be appreciably larger than those of May 18, 1980, and (2) dam operators would again take precautionary steps to lower lake level if Mount St. Helens were to show signs of imminent eruption. Therefore, Swift Reservoir and the downstream lakes (Yale Lake and Lake Merwin) are not considered to be at risk from lahars.

Three natural lakes in the North Fork Toutle River, formed by natural debris dams during the 1980 eruption, have required modifications to their outlets in order to prevent catastrophic outbreaks. The U.S. Army Corps of Engineers provided (1) a tunnel outlet to Spirit Lake, (2) a bedrock spillway channel at Coldwater Lake, and (3) a reinforced spillway channel at Castle Lake to hold the levels of these lakes constant and to prevent them from overtopping their erodible natural dams. A recent study (Roeloffs, 1994), however, has verified earlier conclusions that the natural dam at Castle Lake is potentially susceptible to modes of failure other than overtopping and, under certain conditions, is only marginally stable. Castle Lake contains about 23 million cubic meters (30 million cubic yards) of water and would produce a large lahar if the blockage were to fail. We assume that an outbreak of Castle Lake is a potential hazard.

Mount St. Helens Lakes Menu

Debris Dams and Debris-Dam Lakes Menu

Given the current, relatively quiet, eruptive behavior of Mount St. Helens, debris flows and floods at present constitute the greatest hazards related to volcanic activity. The potential for mudflows and floods was increased by the existence of new ponds and lakes formed when the debris avalanche of May 1980 blocked parts of the preexisting drainage to serve as natural dams. As these natural dams are composed of loose, easily erodible volcanic debris, they are structurally weak and could fail, which would trigger mudflows and floods.

Devastating mudflows or floods or both could be triggered by any or all of the following: heavy rainfall during storms, melting of snow and ice by hot eruptive products (especially pyroclastic flows), or by sudden failure of one of the lakes impounded by the debris avalanche deposits. During winter-the time of peak precipitation and maximum snowpack-the risks of mudflows and floods increase significantly. Normal precipitation in the Mount St. Helens area is heavy, especially on the volcano's upper slopes, where the average annual rainfall totals 140 inches. In a normal winter, the snowpack on the volcano's higher slopes can be about 16 feet thick. Thus, scientists and civil authorities were rightly concerned about the high potential for mudflows and floods, and the Army Corps of Engineers began to take engineering measures-including sediment-retention structures and channel dredging-in the drainages most vulnerable to mudflow and flood hazards.

As an example of the flood hazards in the Mount St. Helens region, in August 1980 the failure of a natural debris dam caused the rapid draining of a 250-acre-feet lake in the Toutle River Valley near Elk Rock. (One "acre-foot" of water is equal to the volume contained in a one-foot layer covering one acre , or about 325 thousand gallons.) The ensuing flood damaged a partially constructed sediment- retention structure and heavy channel-maintenance equipment in the North Fork of the Toutle River. Fortunately, no injuries or deaths resulted. During the next 9 months, no large floods happened, largely because no high-intensity rainfalls occurred even though the total precipitation for the winter and spring of 1980-1981 was near normal. There were no major mudflows or floods the following winter-spring, again because rainfall generally was low intensity. Meanwhile, the levels of the lakes impounded by natural dams, however, gradually rose due to rainfall and runoff.

By the fall of 1982, the debris dams for three of the largest lakes-at Spirit Lake, Coldwater Creek, and South Fork Castle Creek-were becoming substantially filled, thereby increasing the risk of catastrophic flooding should the dams fail or be overtopped. The Corps of Engineers, which in 1981 started construction of controlled outlets at Coldwater and Castle Lakes, began also to control the rise of the level of Spirit Lake by an interim plan of barge-based pumping and discharge into outlet channels.

The USGS and the National Weather Service installed flood-warning systems in the Toutle and Cowlitz River Valleys. By March 1983, Spirit Lake contained 360,000 acre-feet of water, the lake at Coldwater Creek had 67,000 acre-feet, and that at South Fork Castle Creek had 19,000 acre-feet. Scientists and engineers estimated that a breach of the natural dam at South Fork Castle Creek, the smallest of the three lakes, could unleash mudflows and floods comparable to those triggered by the May 18, 1980, eruption of Mount St. Helens. The Corps of Engineers and other Federal, State, and county agencies initiated a variety of projects to mitigate the growing hydrologic hazards. These mitigation projects required many people and much equipment to work in the hazardous zones close to the volcano. To ensure the safety of the mitigation operations, scientists had to intensify their monitoring efforts not only of the volcano itself, but also of the debris-clogged drainage systems.

Though less severe now than in the early 1980s, mudflow and flooding hazards should exist for many years, until such time as the slopes and areas around Mount St. Helens, by revegetation and normal erosion, return to or approach their pre-eruption forest cover, stream gradients, rates of flow, discharge, and channel dimensions. As part of a long-term plan to cope with the continuing hydrologic hazards, the Corps of Engineers, in April 1985, completed the construction of a 1.5 mile-long diversionary tunnel at Spirit Lake. This permanent tunnel system replaced the temporary, barge-based pumping operations to regulate the lake's water level.

Since May 1980, the natural recovery of the drainage system around Mount St. Helens has been substantial. Yet, during this recovery period, some roads in the region sustained significant damage from mudflows and floods, and a number of homes were lost because of stream-bank erosion. How-ever, much more damage would have occurred if it were not for the construction of sediment-retention structures, dredging, and other engineering mitigation measures taken by the Army Corps of Engineers. It should be emphasized, however, the recovering drainage system has not been subjected to a truly major storm during the past decade. Thus, scientists, engineers, and government officials must continue to closely assess and monitor the continuing volcanic and hydrologic hazards. Human efforts to control the floods and sedimentation are designed not only to gain time to lessen the impact of hydrologic hazards until the natural "healing" of the drainage systems around Mount St. Helens is complete, but also to try to guide, if possible, the healing process.

Mount St. Helens Volcano and Hydrologic Hazards Menu

Return to:
[Mount St. Helens Menu] ...
[Mount St. Helens Volcano and Hydrologic Hazards Menu] ...
[Volcano and Hydrologic Hazards Menu] ...