The studies suggest, however, that these possibilities are unlikely in the near future. 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.
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.