DESCRIPTION:
Sediment and Erosion -
Transport and Deposition

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A major problem to people living downstream of Mount St. Helens was the high sedimentation rates resulting from stream erosion of the volcanic deposits. Streams were continuously down cutting channels, eroding their banks, and eating away at the avalanche and lahar deposits. This material was eventually transported downstream and deposited on the streambeds, decreasing the carrying capacity of the channels and increasing the chances of floods.
USGS Photograph taken on February 22, 1982, by Lyn Topinka.
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... Another, less-obvious threat comes from the gradual infilling of river channels by sediment transported from highly erodible, volcanically disturbed landscapes following large eruptions. This sediment can aggrade channel beds with excess sand and gravel for tens to hundreds of kilometers downstream. Such aggradation promotes lateral migration of channels and may cause serious flooding during rainstorms, due to loss of channel capacity necessary to convey floodwaters.

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In order to remove the May 18, 1980 sediment deposits, and to keep up with new sedimentation, the U.S. Army Corps of Engineers began a dredging program on the Toutle (shown here), the Cowlitz, and the Columbia Rivers. By 1987, nearly 140 million cubic yards (110 million meters) of material had been removed from the channels. This is enough material to build twelve lanes of highway, one-foot thick, from New York to San Francisco.
USGS Photograph taken on February 5, 1981, by Lyn Topinka.
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MSH89_sediment_retention_dam_toutle_river_05-89.jpg
In the spring of 1987, construction of a sediment retention dam on the North Fork Toutle River began. This retention dam is designed to help stop the downstream movement of the sediment near where it begins - on the debris avalanche.
USGS Photograph taken in May 1989, by Steve Brantley.
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The May 18, 1980, eruption (of Mount St. Helens) 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. ...

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. ...

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 network of river gages provides 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. ...

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. ...

... 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 ... and emptied nearly 18 million cubic yards of water, mud, and debris into the Swift Reservoir. ...

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. However, 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. ...

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