DESCRIPTION:
Debris Flows, Mudflows, Jökulhlaups, and Lahars

MSH80_mudline_muddy_river_with_USGS_scientist_10-23-80.jpg
Nearly 135 miles (220 kilometers) of river channels surrounding the volcano were affected by the lahars of May 18, 1980. A mudline left behind on trees shows depths reached by the mud. A scientist (middle right) gives scale. This view is along the Muddy River, southeast of Mount St. Helens.
USGS Photograph taken on October 23, 1980, by Lyn Topinka.
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Lahar is an Indonesian word describing mudflows and debris flows that originate from the slopes of a volcano. Both types of flows contain a high concentration of rock debris to give them the internal strength necessary to transport hugh boulders as well as buildings and bridges and to exert extremely high impact forces against objects in their paths. Debris flows are coarser and less cohesive than mudflows. As lahars become dilute in downstream direction they become hyperconcentrated streamflows. Lacking internal strength, the mixture of rock debris and water takes on different flow properties. The coarser debris in this type of flow is no longer held in suspension by matrix strength and therefore settles to the bottom of the flow.

Lahars can be of any size. They may be as small as several centimeters wide and deep, flowing less than one meter per second. Steep, unvegetated slopes during a heavy rain are often good sites to observe such small flows. At the other extreme, they can be a few hundred meters wide, tens of meters deep, flow at several tens of meters per second, and travel over 100 kilometers from a volcano. Such catastrophic lahars are triggered by volcanic eruptions or by massive landslides such as the one that occurred on May 18, 1980, at Mount St. Helens volcano.

Lahars are commonly initiated by:

• large landslides of water-saturated debris,
• heavy rainfall eroding volcanic deposits,
• sudden melting of snow and ice near a volcanic vent by radiant heat or on the flanks of a volcano by pyroclastic flows, or
• breakout of water from glaciers, crater lakes, or from lakes dammed by volcanic eruptions.

Since 1980, lahars have formed by all of these processes at Mount St. Helens.

Volcanic debris flows: mobile mixtures of volcanic debris and water popularly called mudflows often accompany pyroclastic eruptions, if water is available to erode and transport the loose pyroclastic deposits on the steep slopes of stratovolcanoes. Destructive mudflows and debris flows began within minutes of the onset of the May 18 eruption, as the hot pyroclastic materials in the debris avalanche, lateral blast, and ash falls melted snow and glacial ice on the upper slopes of Mount St. Helens. Such flows are also called lahars, a term borrowed from Indonesia, where volcanic eruptions have produced many such deposits.

A debris flow (sometimes called mudflow) is a flowing mixture of water-saturated debris that moves downslope under the force of gravity. Debris flows consists of material varying in size from clay to blocks several tens of meters in maximum dimension. When moving, they resemble masses of wet concrete and tend to flow downslope along channels or stream valleys. Debris flows are formed when loose masses of unconsolidated wet debris become unstable. Water may be supplied by rainfall, by melting of snow or ice, or by overflow of a crater lake. Debris flows may be formed directly if lava or pyroclastic flows are erupted onto snow and ice. Debris flows may be either hot or cold, depending on their manner of origin and temperature of their constituent debris.

Debris flows can travel great distances down valleys, and debris-flow fronts can move at high speeds -- as much as 85 kilometers per hour. Debris flows produced during an eruption of Cotopaxi volcano in Ecuador in 1877 traveled more than 320 kilometers down one valley at an average speed of 27 kilometers per hour (Macdonald, 1972). High-speed debris flows may climb valley walls on the outsides of bends, and their momentum may also carry them over obstacles. Debris flows confined in narrow valleys or by constructions in valleys can temporarily thicken and fill valleys to heights of 100 meters or more (Crandell, 1971).

The major hazard to human life from debris flows is from burial or impact by boulders and other debris. People and animals also can be severely burned by debris flows carrying hot debris. Buildings and other property in the path of a debris flow can be buried, smashed, or carried away. Because of their relatively high density and viscosity, debris flows can move and even carry away vehicles and other objects as large as bridges and locomotives.

Because debris flows are confined to areas downslope and downvalley from their points of origin, people can avoid them by seeking high ground. Debris-flow hazard decreases gradually downvalley from possible source volcanoes but more abruptly with increasing altitude above valley floors. People seeking to escape flows should climb valley sides rather than try to outrun debris flows in valley bottoms. During eruptive activity or precursors to eruptions, local government officials may ask form prompt evacuation of areas likely to be affected.

Lahars (Debris Flows or Mudflows) are mixtures of water, rock, sand, and mud that rush down valleys leading away from a volcano. They can travel over 50 miles downstream, commonly reaching speeds between 20 and 40 miles per hour. Sometimes they contain so much rock debris (60-90% by weight) that they look like fast-moving rivers of wet concrete. Close to the volcano they have the strength to rip huge boulders, trees, and houses from the ground and carry them downvalley. Further downstream they simply entomb everything in mud. Historically, lahars have been one of the most deadly volcanic hazards.

Lahars can form in a variety of ways, either during an eruption or when a volcano is quiet. Some examples include the following: (1) rapid release of water from the breakout of a summit crater lake; (2) generation of water by melting snow and ice, especially when a pyroclastic flow erodes a glacier; (3) flooding following intense rainfall; and (4) transformation of a volcanic landslide into a lahar as it travels downstream.

Lahars (also called volcanic debris flows or mudflows) are mixtures of water-saturated rock debris that flow downslope under the force of gravity. ... Rock debris in lahars ranges in size from clay to blocks several tens of meters in maximum dimension. When moving, lahars resemble masses of wet concrete and tend to be channeled into stream valleys. Lahars are formed when loose masses of unconsolidated, wet debris become mobilized. ...

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Debris flow at Tahoma Creek, July 26, 1988
-- USGS Photo by G. G. Parker, USGS/WRD/Tacoma

... The smallest, but most frequent, debris flows at Mount Rainier begin as glacial outburst floods, also called by the Icelandic term "jokulhlaup" (pronounced "yo-kul-h-loip"). Outburst floods at Mount Rainier form from sudden release of water stored at the base of glaciers or within the glacier ice. ...

Outburst floods become debris flows by incorporating large quantities of sediment from valley floors and walls, often by triggering landslides that mix with the flood waters. The transformation from water flood to debris flow occurs in areas where streams have eroded glacially derived sediments and sediment-rich, stagnant glacier ice that was stranded in valleys as glaciers thinned and retreated earlier in this century. ...

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Mount Hood and White River Drainage, with 1998 Flow Deposits
-- USGS Photo by Cynthia Gardner, September 4, 1998

Jökulhlaups (glacial-outburst floods) have been recorded from the Zigzag, Ladd, Coe, and White River Glaciers. In 1922, a dark debris flow issued from a crevasse high on Zigzag Glacier and moved 650 meters over the ice before entering another crevasse; this event initiated a scare that Mount Hood was erupting (Conway, 1921). The Ladd Glacier jökulhlaup in 1961 destroyed sections of the road around the west side of the mountain and partly undermined a tower of a major powerline (Birch, 1961). The Coe Glacier outburst occurred around 1963, causing a section of trail to be abandoned and the "round-the-mountain" trail to be rerouted farther from the glacier. Jökulhlaups from White River Glacier were reported in 1926, 1931, 1946, 1949, 1959, and 1968; the Highway 35 bridge over the White River was destroyed during each episode. The more frequent outbursts from White River Glacier may be due in part to an increase in size of the fumarole field at the head of the glacier at Crater Rock (Cameron, 1988).

Mount Hood Mudflows and Lahars Menu

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Debris flow at Tahoma Creek, July 26, 1988
-- USGS Photo by G. G. Parker, USGS/WRD/Tacoma

... The smallest, but most frequent, debris flows at Mount Rainier begin as glacial outburst floods, also called by the Icelandic term "jokulhlaup" (pronounced "yo-kul-h-loip"). Outburst floods at Mount Rainier form from sudden release of water stored at the base of glaciers or within the glacier ice. ...

Outburst floods become debris flows by incorporating large quantities of sediment from valley floors and walls, often by triggering landslides that mix with the flood waters. The transformation from water flood to debris flow occurs in areas where streams have eroded glacially derived sediments and sediment-rich, stagnant glacier ice that was stranded in valleys as glaciers thinned and retreated earlier in this century. ...

Glacier-generated debris flows at Mount Rainier travel downstream at speeds of 5-10 meters per second (10-20 miles per hour) or more. ... These flows typically have steep, bouldery snouts--up to 10-20 meters (30-60 feet) high in the most constricted parts of a stream valley--followed by a churning mass of mud, rock, and vegetation. Their deafening noise is often accompanied by strong local wind, thick dust clouds, and violent ground shaking.

Debris flows usually follow stream channels and construct their own levees as they move, but their exact paths are unpredictable. As a debris flow moves downstream from Mount Rainier's steep flanks onto relatively gentle slopes, the flow's bouldery snout may clog the stream channel; the moving mass behind the snout may then overtop the banks and cut a new channel, perhaps through forest or across trails and roads. Debris flows at Mount Rainier typically come to rest after perhaps 30 minutes to an hour, leaving muddy, bouldery deposits from which muddy water drains for a period of a few hours to a few days. ...

The largest debris flows at Mount Rainier are unrelated to glacial outburst floods. Several times during the last 6000 years, debris flows enormously larger than any caused by outburst floods were triggered by huge rock avalanches and travelled far beyond the park boundaries.

Mount Rainier Mudflows and Lahars Menu

Post-glacial deposits at Mount Rainier are dominated by lahars; over 60 have been identified. Although relations between Holocene tephra and flowage deposits remain speculative, at least some lahars were probably eruption induced, most notably the Paradise lahar and the Osceola Mudflow, which has been dated at 5,040 Carbon-14 years B.P., had a volume >10^9 cubic meters, and a profound geomorphic effect on the Puget Sound shoreline, over 100 kilometers from the mountain. ... Wood from buried trees in the Round Pass Mudflow has been dated at 2,600 Carbon-14 years B.P., ... the Electron Mudflow has been dated at 530 Carbon-14 years B.P. This lahar, which evidently began as a failure of part of the western edifice, has not been correlated with any eruptive activity at Mount Rainier and may have occurred without precursory eruptive phenomena. ...

Mount Rainier Historical Mudflows Menu

MSH80_mudline_muddy_river_with_USGS_scientist_10-23-80.jpg
Nearly 135 miles (220 kilometers) of river channels surrounding the volcano were affected by the lahars of May 18, 1980. A mudline left behind on trees shows depths reached by the mud. A scientist (middle right) gives scale. This view is along the Muddy River, southeast of Mount St. Helens.
USGS Photograph taken on October 23, 1980, by Lyn Topinka.
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(On May 18, 1980) ... The collapse of the north flank ... (of Mount St. Helens)... produced the largest landslide-debris avalanche recorded in historic time. ... Part of the avalanche surged into and across Spirit Lake, but most of it flowed westward into the upper reaches of the North Fork of the Toutle River. ... The resulting hummocky avalanche deposit consisted of intermixed volcanic debris, glacial ice, and, possibly, water displaced from Spirit Lake. Covering an area of about 24 square miles, the debris avalanche advanced more than 13 miles down the North Fork of the Toutle River and filled the valley to an average depth of about 150 feet; the total volume of the deposit was about 0.7 cubic mile. ...

Destructive mudflows and debris flows began within minutes of the onset of the May 18 eruption, as the hot pyroclastic materials in the debris avalanche, lateral blast, and ash falls melted snow and glacial ice on the upper slopes of Mount St. Helens. Such flows are also called lahars, a term borrowed from Indonesia, where volcanic eruptions have produced many such deposits.

Mudflows were observed as early as 8:50 a.m. PDT in the upper reaches of the South Fork of the Toutle River. The largest and most destructive mudflows, however, were those that developed several hours later in the North Fork of the Toutle River, when the water-saturated parts of the massive debris avalanche deposits began to slump and flow. The mudflow in the Toutle River drainage area ultimately dumped more than 65 million cubic yards of sediment along the lower Cowlitz and Columbia Rivers. The water-carrying capacity of the Cowlitz River was reduced by 85 percent, and the depth of the Columbia River navigational channel was decreased from 39 feet to less than 13 feet, disrupting river traffic and choking off ocean shipping.

Mudflows also swept down the southeast flank of the volcano-along the Swift Creek, Pine Creek, and Muddy River drainages and emptied nearly 18 million cubic yards of water, mud, and debris into the Swift Reservoir. The water level of the reservoir had been purposely kept low as a precaution to minimize the possibility that the reservoir could be overtopped by the additional water-mud-debris load to cause flooding of the valley downstream. Fortunately, the volume of the additional load was insufficient to cause overtopping even if the reservoir had been full.

On the upper steep slopes of the volcano, the mudflows traveled as fast as 90 miles an hour; the velocity then progressively slowed to about 3 miles an hour as the flows encountered the flatter and wider parts of the Toutle River drainage. Even after traveling many tens of miles from the volcano and mixing with cold waters, the mudflows maintained temperatures in the range of about 84 to 91 degrees (F); they undoubtedly had higher temperatures closer to the eruption source. Shortly before 3 p.m., the mud- and debris-choked Toutle River crested about 21 feet above normal at a point just south of the confluence of the North and South Forks. Another stream gage at Castle Rock, about 3 miles downstream from where the Toutle joins the Cowlitz, indicated a high-water (and mud) mark also about 20 feet above normal at midnight of May 18. Locally the mudflows surged up the valley walls as much as 360 feet and over hills as high as 250 feet. From the evidence left by the "bathtub-ring" mudlines, the larger mudflows at their peak averaged from 33 to 66 feet deep. The actual deposits left behind after the passage of the mudflow crests, however, were considerably thinner, commonly less than 10 percent of their depth during peak flow. For example, the mudflow deposits along much of the Toutle River averaged less than 3 feet thick.

Mount St. Helens Mudflows and Lahars Menu

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Armero, Colombia, destroyed by lahar on November 13, 1985.More than 23,000 people were killed in Armero when lahars (volcanic debris flows) swept down from the erupting Nevado del Ruiz volcano. When the volcano became restless in 1984, no team of volcanologists existed that could rush to the scene of such an emergency. However, less than a year later, the U.S. Geological Survey organized a team and a portable volcano observatory that could be quickly dispatched to an awakening volcano anywhere in the world.
USGS Photo by R. J. Janda, 1985

Volcanic debris flows (mudflows or lahars): Flowing mixture of water-saturated debris, intermediate between a debris avalanche and a water flood, typically moves at speeds of several tens of miles per hour on steep slopes, slowing to less than 10 miles per hour on gentle slopes. Debris flows can travel tens of miles down valley and devastate distant unsuspecting communities, as in Colombia during the 1985 eruption of Nevado del Ruiz.

Nevado del Ruiz Menu

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