English: Map text:

Hundreds of years ago, an entire hillside along the Columbia River collapsed in a massive landslide covering an area of about six square miles. The Bonneville landslide was so huge that it created a natural dam in the Columbia River, forming a temporary lake stretching all the way to Wallula Gap, 150 miles upriver. The deposit is known as the “Bridge of the Gods”, because it temporarily connected the land on both sides of the Columbia River. The modern-day bridge at Cascade Locks is named after this prehistoric land bridge.

The Bonneville landslide’s headscarp is exposed on Table Mountain, which has an almost vertical 800-foot-tall open face close to its peak. Table Mountain is composed of Columbia River Basalt (CRB) lava flows, that sit on top of the sedimentary Eagle Creek Formation. The sedimentary rock weathers into clay as water trickles down through cracks in the CRB lavas. The Bonneville landslide, like many of the large landslides in the Cascade landslide complex, was activated when CRBs slid along this weak clay layer.

When did this huge landslide happen? Using methods such as radiocarbon and tree-ring dating, scientists can figure out the timing of a geologic event. The most recent investigations tell us that the Bonneville landslide occurred in the mid-1400s. Areas of the Cascade landslide complex are still moving today. A satellite mapping technique known as InSAR measures small movements on Earth’s surface. Using this technology, researchers determined that a section of the Cascade landslide complex had moved 2.3 feet downslope between 2007 and 2011. That may not sound like a lot, but it tells us that this area is still in motion. This movement could impact structures built on the landslide.

Map by Daniel E. Coe, Washington Geological Survey, Washington State Department of Natural Resources.

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