Pine Creek Bridge Vicinity

Pine Creek drains an area of about 24 square miles and heads on the south flank of Mount St. Helens. Voluminous lahars associated with the 1980 eruption flowed down the creek. It took approximately 28 minutes for the flow, averaging 34 feet per second in velocity, to traverse the 14.8 miles from the cone. Lahars 32 feet deep flowed past the bridge; mudlines are still visible on trees upstream. A boulder estimated to weigh about 60 tons was found resting on the road surface, only the bottom 3 feet buried in lahar deposits. The boulder has been moved to permit work on the road and bridge. At the bridge, more bank erosion occurred from storm runoff in late 1980-83 than from the May 18, 1980 lahars.

Excerpt from: Doukas, 1990, Road Guide to Volcanic Deposits of Mount St. Helens and Vicinity, Washington: USGS Bulletin 1859, 53p.

Pine Creek Bridge

Adjacent to the small turnout on the west side of the Pine Creek Bridge is a 37-ton boulder that was deposited in the old roadway 33 feet above the creek by the May 18, 1980, lahar. This lahar was generated by the pyroclastic surge that descended the east slope of Mount St. Helens and swept across Muddy fan. Older material in the pit on the east side of the bridge and visible upstream in the northeast bank of Pine Creek is composed of pyroclastic-flow and lahar deposits of the Pine Creek eruptive period (3,000-2,500 years B.P.). Abundant alders and other pioneer plant species have revegetated the banks of Pine Creek since 1980.

Excerpt 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