DESCRIPTION:
Mount St. Helens Snowpack and Ice Accumulation

Mount St. Helens is located on the west side of the Cascade Range in southern Washington. Average annual precipitation in the Toutle River drainage basin varies from about 1,100 millimeters (45 inches) in the valleys near sea level, to more than 3,500 millimeters (140 inches) above 1,000 meters (3,300 feet). Most precipitation occurs during a well-defined winter rainy season which begins about October, peaks in December or January, and declines into the spring. During this period, precipitation usually occurs as light to moderate intensity rain, rather than as heavy rains. Freezing level may vary from sea level to above 1,000 meters (3,300 feet) throughout the winter. Midwinter snow line typically lies between about 750 - 1,200 meters (2,500 - 4,000 feet) above sea level. Snow accumulation varies from zero at lower elevations to more than 7.6 meters (25 feet) above 1,500 meters (5,000 feet). Density of the snowpack increases from about 25 to 45 percent water equivalent between early winter and April (Pacific Northwest River Basin Commission, 1970). The largest flows (both pre-and post eruption) occur when relatively warm, heavy rains fall on snow, and snow melt supplements the rainfall runoff.

A large volume of snow and ice is presently accumulating in the Mount St. Helens crater, protected by the shade of the high, steep crater walls. This accumulation provides a growing potential water source for lahars in the North Fork Toutle River valley. It is already mixed with rock debris eroded from the crater walls, and this debris would augment the formation of a lahar. It is possible that a large eruption could melt most or all of this snow and ice in a matter of tens of minutes. A very small eruption in 1982 rapidly melted enough snow and ice in the crater to trigger a 4 million cubic meters (5.2 million cubic yards) flood that transformed into a lahar and flowed all the way to the Cowlitz River. At the present time (1995), about 53 million cubic meters (70 million cubic yards) of snow and ice has accumulated. If completely melted, this would produce about 38 million cubic meters (50 million cubic yards) of water. At the present rate of accumulation, the volume of snow and ice will double in about 15 years.

Permanent and seasonal snow and ice also blanket the outer flanks of Mount St. Helens. A sufficient volume exists there in winter or spring to produce flank lahars similar in magnitude to those of May 18, 1980, if another large eruption were to occur. Lahars formed on the outer flanks can be expected to be substantially smaller than flows generated in the crater.

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