| Hazards and Monitoring |
| Sediment Erosion, Transport, and Deposition |
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.
| Water and Sediment Discharge Measurements |
Thirteen 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.
| River-Channel Surveys |
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 Monitoring |
The debris avalanche raised the level of Spirit Lake 64 meters and dammed its natural outlet even higher. ... and several lakes formed in tributaries dammed by the avalanche; the largest lakes formed in the tributaries of Coldwater and Castle Creeks. ... 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 ...
| Lake Gages |
Six 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.
| Stability Studies |
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.
| Research |
The 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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