| The catastrophic eruption of Mount St. Helens on May 18, 1980, resulted in the emplacement of about 2.8 cubic kilometers of volcanic products throughout much of the drainage basin of the Toutle River in Washington. Between 115 and 155 million tons of sediment were discharged from the Toutle River to the Cowlitz River during May 18-19, 1980. An additional 190 million tons of sediment were transported out of the Toutle River Basin between 1981 and 1992. Another 40 million tons were trapped behind the permanent sediment-retention structure on the North Fork Toutle River. Sediment yields from the upper North ForkToutle River were among the highest recorded, ranging from 18,000 tons per square kilometer in 1989 to about 500,000 tons per square kilometer in 1980. By 1992, only about 8 percent of the material deposited in the debris avalanche had been transported out of the Toutle River system. -- Simon, 1999 |
