Lahars Caused by Lake Breakouts

Castle Lake, Mount St. Helens, Washington

Photograph by S. Brantley
September 29, 1992

Lake breakouts commonly occur weeks to months after a river or one of its tributaries become blocked by a landslide or other volcanic deposits, especially pyroclastic flows and lahars. The most frequent cause of a lake breakout is the overflow of water across the newly formed dam and subsequent erosion and rapid downcutting into the loose rock debris. As more water rushes from the lake, the initial channel grows deeper and wider, which allows even more water to surge downstream. By eroding the blockage and river channel downstream, the initial surge of water will incorporate a tremendous volume of sediment and increase in volume two to four times or more as it races downvalley.

Castle Lake, Mount St. Helens, Washington

Blocked by the massive landslide that slid from Mount St. Helens during an eruption on May 18, 1980, Castle Creek quickly filled with water to form a lake (center left). In 1981, concern among scientists and public officials about the possibility of a sudden lahar caused by the breakout of water from Castle Lake led the U.S. Army Corps of Engineers to excavate an outlet channel and line it with heavy wire mesh and boulders. The new channel prevented the lake from overtopping its new debris dam. The water level and stability of the blockage is still monitored by scientists of the U.S. Geological Survey.

Lake breakout causes lahars at Mount Pinatubo, Philippines

Hundreds of lahars triggered by heavy rain formed after the 1991 eruption of Mount Pinatubo. The rainwater eroded loose, hot pyroclastic flow deposits (shown here) that filled river valleys around the volcano to depths of 220 meters. The lahars not only covered large areas downstream with sediment,destroying homes and farmland, but also temporarily blocked tributary streams.

Pasig-Portrero River, Philippines

July 5, 1994

Lahars Block Tributary

Pyroclastic-flow deposits from the 1991 eruption filled the south fork of the Pasig-Portrero River, which flows from right to left. After the formation and breakout of several lakes from this valley, a new lake began to form in 1994 during the Philippine's summer monsoon season.
Lake formed by lahar deposits along Pasig-Portrero River, Philippines

August 30, 1994

Lake Forms Behind Lahar Deposits

Full development of the lake. The primary cause of this new lake was the deposition of sediment by lahars along the main channel of the river during the 1994 rainy season. The sediment prevented water flowing into the south fork from entering the main river.
Lake breakout begins

September 22, 1994

Lake Breakout Generates Lahar

After a moderate rainfall, water flowing over the blockage severely eroded through the lahar and pyroclastic-flow deposits during the night. The resulting lake breakout generated a lahar that swept downstream and killed approximately 25 people.
Bacolor, Philippines

July 27, 1994

Community Before Lake Breakout

Town of Bacolor (foreground) along the levee-bounded Pasig-Portrero River, downstream of the lake. The gray area in the upper left is covered with sediment deposited by lahars and normal streamflow in 1992. New lahar outbreaks through the levee in July 1994 had begun to encroach at the top right.
Bacolor after lake breakout

September 6, 1995

Community After Lake Breakout

Lahars from September 1994 though and after the date of this photograph buried Bacolor to depths of 5 meters in the town and more than 10 meters in some outlying villages.