Each week we will feature a new landslide in Washington State. Washington State is covered with dynamic and sometimes quirky landslides.

Bonneville Landslide, Skamania County

The Bonneville landslide is perhaps the second most well known landslide in Washington State, following the 1980 Debris Avalanche of Mt. St. Helens. The landslide was probably first recognized by the Klickitat Nation (also known as the Qwû’lh-hwai-pûm, or loosely translated as prairie people). The landslide is known as the Bridge of the Gods and served as a water crossing for trade routes across the Columbia River. The Bridge of the Gods has many stories of its origin, perhaps the most well known is the story by the Klickitat people.

From Wikipedia:
“Native American lore contains numerous legends to explain the eruptions of Mount St. Helens and other volcanoes in the Cascade Volcanic Arc. The most famous of these is the Bridge of the Gods legend told by the Klickitats. In their tale, the chief of all the gods, Tyhee Saghalie and his two sons, Pahto (also called Klickitat) and Wy’east, traveled down the Columbia River from the Far North in search for a suitable area to settle.

They came upon an area that is now called The Dalles and thought they had never seen a land so beautiful. The sons quarreled over the land and to solve the dispute their father shot two arrows from his mighty bow; one to the north and the other to the south. Pahto followed the arrow to the north and settled there while Wy’east did the same for the arrow to the south. Saghalie then built Tanmahawis, the Bridge of the Gods, so his family could meet periodically.

When the two sons of the Saghalie fell in love with a beautiful maiden named Loowit, she could not choose between them. The two young chiefs fought over her, burying villages and forests in the process. The area was devastated and the earth shook so violently that the huge bridge fell into the river, creating the Cascades Rapids of the Columbia River Gorge.

For punishment, Saghalie struck down each of the lovers and transformed them into great mountains where they fell. Wy’east, with his head lifted in pride, became the volcano known today as Mount Hood and Pahto, with his head bent toward his fallen love, was turned into Mount Adams. The fair Loowit became Mount St. Helens, known to the Klickitats as Louwala-Clough which means “smoking or fire mountain” in their language (the Sahaptin called the mountain Loowit).”

The story captures two important keys to the landslide. A strong shaking and burying of forests and villages. Early research into the Bonneville Landslide places the landslide movement around 1,100AD. However, a recent find of a buried douglas fir log about 150 feet deep in the landslide mass places the landslide failure somewhere between 250 and 400 years ago. This conceivably places the Bonneville Landslide at the 1700 Cascadia Earthquake. This would follow the story of the Klickitat People or great shaking and landslide movement. When the landslide moved, it completely blocked the Columbia River, creating a temporary dammed lake. The waters would have slowly cut through the material, reestablishing the channel.

Cascade Landslide Complex

The Bonneville Landslide is part of a larger complex of landslides. The ages are not well constrained and may have occurred with episodic movement through time. The two landslides that are most recent are the Bonneville Landslide (probably failed around 1700) and the Carpenters Lake Landslide (Age Unknown).

Cascade Landslide Complex

Cascade Landslide Complex

The two smaller landslides, the Greenleaf Basin and the Piper Road Landslides are more recent, smaller landslides.

Geology
The Cascade Landslide is composed mostly of debris of the Yakima Basalt, Eagle Creek Formation, and a platy olivine basalt flow (Wise, 1961). The landslide is suggested to have a depth of about 300 to 400 feet and slides on a clay layer at the base of the Eagles Creek Formation. The units are gently dipping southward, which probably adds to the instability of the area.

Current Hazards
The Cascade Landslide Complex is perhaps one of the more dangerous landslides in Washington State. The Bonneville Dam sits on the landslide debris of the Bonneville Landslide. Major pipelines, powerlines, and transportation routes cross the landslide. Future movements (potentially during a Cascadia Subduction Earthquake) would result in major disruptions in utilities and the potential for the redamming of the Columbia River. The other problem, it might not occur in the Cascade Landslide Complex at all, large landslides dot the area surrounding this area and some might have the potential to also partially or completely block the Columbia River. A damming of the Columbia River would be devastating. The waters would form into a lake, flooding low lying communities, further impacting transportation routes and utilities. The potential exists for the Columbia to then rupture the dam, creating a massive damburst flood. That flood has one major target in its way, Portland. Although this is a very extreme example (and the odds for the events to occur this way are very slim), a large debris flood through the Columbia River would cause unprecedented damage, especially for a landslide. However, chances are future landslide movements would be less extreme, although the likely major disruption of utilities and transportation would be felt throughout western Washington and Oregon.

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I guess I am still catching up on landslides in the news. The Greenleaf Basin Landslides occurred sometime near or before January of 2008.

Greenleaf Basin Landslide in the Bonneville Landslide

Greenleaf Basin Landslide in the Bonneville Landslide

The landslide is within the scarp of the Bonneville Landslide (the famous Bonneville landslide that formed the Bridge of the Gods, coincidentally, and not to be confused from, the Bridge of the Gods, which connects Highway 14 to Highway 84 – Washington to Oregon). The Bonneville Landslide failed probably in conjunction with the 1700 Cascadia Earthquake.

Greenleaf Basin Landslide - Prefailure View

Greenleaf Basin Landslide - Prefailure View

The above orthophoto is a 2006 photo of the area where the Greenleaf Basin Landslide occurred. The landslide cut into the scarp and incorporated numerous mature trees (probably over 50 years old).

(The photos are from Don Nelson)

Landslide in distance, note the scarp of the Bonneville Landslide Complex

Landslide in distance, note the scarp of the Bonneville Landslide Complex

View of the scarp and material from the landslide

View of the scarp and material from the landslide

A clue to why this area is unstable, look at those springs!

A clue to why this area is unstable, look at those springs!

A good view of the woody debris that came down with the material, people for scale (in red)

A good view of the woody debris that came down with the material, people for scale (in red)

A closer view of the springs and change in lithology

A closer view of the springs and change in lithology

A close up view of the scarp

A close up view of the scarp

As you can tell in the pictures, the lithology of the landslide probably helped to create an unstable environment. The top layer is probably a Quaternary basaltic andesite flow (referred to as the basaltic andesite of the Cascade Landslide) which is on the Eagles Formation (a Miocene continental conglomerate) The springs are probably representing a perched layer of water and in December of 2007, we had quite a bit of rain. The average rainfall for the Bonneville area is 12.72 inches (for the month of December). In 2007, the rainfall was 19.45 inches. The beds are gently dipping to the south (most of the photos are looking north and the dip isn’t very apparent in them). The increase in pore pressure along the contact probably help to pry away the upper rock and combined with the dip, the mass of rock lost what resistive forces it had to stay in place.

An interesting side note. The layering of an incompetent over competent lithology, combined with a dipping bed, is probably a major component why the Bonneville Landslide Complex exists where it is. The Stevenson Landslide (also known as the Rock Creek or Piper Road Landslide) is also on a similar situation.