Swift Reservoir Overlook

This overlook is a good place to stop and view geographic features constructed during several stages of Mount St. Helens. The terrace on which the parking lot is situated is the remnant of a debris fan constructed during the Cougar eruptive stage (approximately 20,000 years ago). A dissected part of the fan is visible on the south valley wall downstream. Underlying this fan is a very old Mount St. Helens lahar deposit of the Ape Canyon eruptive stage (older than 36,000 years ago). Holes drilled into the valley bottom near the Swift Reservoir dam site penetrated an even older lahar deposit whose base is 160 feet below the surface.

If you look to te north-northeast, you can see Marble Mountain, a shield volcano composed of three scoria cones. An andesite cone on the south flank of Marble Mountain was built during the last eruption from the shield about 160,000 years ago. Marble Mountain is part of a zone of volcanoes that extend from Mount St. Helens to the south more than 36 miles toward the Columbia River.

Excerpts from: Pringle, 1993, Roadside Geology of Mount St. Helens National Volcanic Monument and Vicinity: Washington Department of Natural Resources, Division of Geology and Earth Resources Information Circular 88

Swift Reservoir and Dam

Swift Reservoir is formed by a rock and earthfill dam. Storage began September 19, 1958; the dam was completed in December 1958. Usable capacity: 446,600 acre-feet between elevations 878 feet (lower limit for economic operation), and 1,000.5 feet (maximum operating limit). Dead storage is unknown. Water is used by PacifiCorp for power development.

Excerpt from: USGS Water-Data Report WA-97-1