DESCRIPTION:
Monitoring Mount St. Helens Debris-Dam Lakes

The debris avalanche raised the level of Spirit Lake 64 meters and dammed its natural outlet even higher. Many small ponds filled closed depressions on top of the avalanche deposit, and several lakes formed in tributaries dammed by the avalanche; the largest lakes formed in the tributaries of Coldwater and Castle Creeks. In late 1980, some of the ponds overtopped and swiftly eroded their new outlets. The rapid release of water generated highly erosive flows on the avalanche and transported large volumes of sediment down the lower reaches of the Toutle and Cowlitz Rivers.

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 of Coldwater and Castle Lakes, and water from Spirit Lake is currently being pumped into the Toutle River by the Corps of Engineers as a temporary measure to control its level (Web note: 1984 publication). Permanent solutions are being considered to alleviate the flood threat from Spirit Lake.

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.

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.

[Image,80K,JPG]
Outlet channels have been built at Castle Lake and Coldwater Lake (shown here) to stabilize water levels and prevent overtopping of the debris dams.
-- USGS Photo by Lyn Topinka, October 17, 1981

[Image,143K,GIF]
Aerial view, Spirit Lake pump station. From late 1982 through the spring of 1985, a pump station operated at Spirit Lake to maintain that lake at a safe level. Without pumping, Spirit Lake was estimated to overtop its debris dam within a year.
-- USGS Photo by Lyn Topinka, September 23, 1983

[Image,94K,JPG]
In the spring of 1985 a permanent tunnel was opened, allowing water to drain out of the lake safely. This tunnel is 11 feet (3.4 m) in diameter and more than 1.5 miles (2.5 km) in length. The water level of Spirit Lake is now maintained at approximately 100 feet (30 m) below the estimated overtopping level.
-- USGS Photo by Lyn Topinka, October 2, 1986


MORE Mount St. Helens Photo Archives - Lakes and Reservoirs

Return to:
[Volcano and Hydrologic Monitoring Menu] ...
[Hydrology and Hydrologic Processes Menu] ...
[Natural Debris-Dam Lakes Menu] ...