Oso Slide, Part of a Bigger Picture

A rock and mud slide along the Mountain Loop Highway

    A water-logged hillside in the Cascade foothills east of tiny Oso collapsed Saturday morning, sending a mile-wide wall of mud and debris crashing over a riverfront neighborhood. It buried the highway and dammed the North Fork Stillaguamish River. Horrified, we’re watching rescue efforts, along with the rest of the nation.

    We live in a region prone to such disasters because of unstable soils left by ice-age glacial events. Just this morning a newspaper article mentioned an 1820 tsunami caused when a headland on Camano Island’s south end collapsed into Puget Sound. Three hundred years ago, a mudslide near Lake Ozette on Washington’s coast buried and preserved a Makah Indian village. It was rediscovered in the 1960s when a fierce winter storm eroded the shoreline to reveal long-hidden artifacts.

    Smaller slides halt railroad trains every winter along the Sound between Everett and Seattle. We see slides along nearby Pioneer Highway, cutting into the bluff bordering the Skagit Delta farmlands.

    I remember a devastating slide in 1950 at Gold Basin, in the valley of the South Fork Stillaguamish where I grew up. That fork flows through Verlot and past Granite Falls before joining the North Fork at Arlington. Then the combined rivers make their way through the lowland valley to salt water near Stanwood.  Geologists tell us that historically as well as recently, slides are common in both the North Fork and South Fork valleys. Though the Gold Basin slide did not result in loss of life, it changed and is still changing the land.

    Both forks of the Stillaguamish share a geologic history with other river valleys in the Western Cascade Mountains. The river basins of the western Cascades begin as narrow, steep-sided mountain valleys whose floors are generally less than 3000 feet above sea level. The rivers flow into broad lowland alluvial river valleys, through flood plains to the extensive glacial outwash plain along Puget Sound. 

    The Puget Lobe of the Cordilleran Ice Sheet came down out of the north, as far as Olympia. It advanced, then retreated many times, shaping the geography of this region. Old channels and terraces that mark the retreat of the ice margins are still visible. As the ice pressed southward, smaller glaciers filled the upper river basins. Only the tops of peaks like Mt. Pilchuck protruded from the ice. The smaller glaciers scoured away the mountainsides. They pulverized solid rock into flour-fine clay and left bowl-shaped amphitheaters called “cirques.”

     About 14,000 years ago, the glaciers filling the lower mountain valleys melted. But remnants of the big ice sheet still plugged the valley outlets, blocking drainage. Long lakes formed in the valleys as the alpine glaciers melted and dropped deposits of rocks, gravel and lighter sand and clay. Finally, the retreating Puget Lobe allowed rivers like the Stillaguamish and the Skagit to find their way to the sea, carrying tons of silt along with them to form the rich soils found in the Skagit-Stillaguamish deltas.

SkagitValley clay soils, once part of the mountains in the distance

Dad and little brother David beneath the Gold Basin cliffs, c. 1947

     Old lake deposits are visible at many places, wherever the river cuts through embankments of sand, clay or gravel. Some are very deep, like the hillside that collapsed near Oso or the cliffs at Gold Basin State Park.  At Gold Basin, the forest-crowned palisades, layered with fine tan dirt and gray silty clay, towered nearly straight up from the river’s edge.  Fans of crumbled sand and gravel scalloped their base. In places, water seeped between layers of permeable and less permeable materials to streak the face of the cliffs.

65 years later, showing trees growing on slide and new location of river

    The winter of 1950, we had a lot of rain. It saturated the soil to a great depth and sent the river roaring toward Port Susan Bay. Water percolating down through the layered deposits at Gold Basin hit less permeable material. The water moved out toward the face of the cliff. And the waterlogged soil above the interface began to move with it. The cliff and its crown of trees collapsed into the river. With no place else to go, the river forced its way into the park, chewing away great chunks of the opposite bank and following the lines of least resistance to gouge temporary channels along and across what had been park roads. The sparkling waters of the Stillaguamish turned an opaque gray-brown. For years, banks and boulders wore a slick coating of clay.

    More than fifty years later, the bluffs at Gold Basin are still unstable. The river eats away at the bottom, dissolving the soil into gray fluid and making life difficult for fish and the creatures they live on. Below the slide, new forest has grown on the island that formed where the river once flowed.

    Storms and the river still combine to challenge those who live along the river and those who attempt to keep it healthy for the wild things that live in and near it.

    More recently, in October, 2003, huge amounts of rain fell throughout the northwestern Cascade Mountains. Back country trails washed out or were obliterated by slides, bridges crossing mountain streams were damaged, and sections of the old Mountain Loop Highway between Barlow Pass and Darrington crumbled into the Sauk and disappeared. People living near the Stillaguamish below Granite Falls found the river racing through their houses.

    The hill that collapsed on March 22 has been the focus of concerns since the 1950s. The Seattle Times refers to a report filed in 1999 with the U.S. Army corps of Engineers, warning of “the potential for a large catastrophic failure.” The report was written by Daniel and Lynne Rodgers Miller.  “We’ve known it would happen at some point,” Mr. Miller said. “We just didn’t know when.”

    A lidar map is made by lasers which can visually strip away vegetation to reveal the underlying landforms. The Times published one such map that shows the semicircular scars of many past slides in the area of Saturday’s disaster.

    There were slides in 2003 and in 2006 on the same hillside, yet homes continued to be built in the path of danger. All of us should remember that it was Nature’s job to sculpt and change the land long before human history began, and she will continue to do so, regardless of our wishes.


The river sculpts the land.