Over the last several thousand years a series of large landslides have occurred on the north side of the Columbia River Gorge , sending massive amounts of debris south from Table Mountain and Greenleaf Peak into the gorge near the present site of Bonneville Dam . The most recent and significant is known as the Bonneville Slide , which formed a massive earthen dam , filling of the river 's length . Various studies have placed the date of the Bonneville Slide anywhere between 1060 and 1760 AD ; the idea that the landslide debris present today was formed by more than one slide is relatively recent and may explain the large range of estimates . It has been suggested that if the later dates are accurate there may be a link with the 1700 Cascadia earthquake . The pile of debris resulting from the Bonneville Slide blocked the river until rising water finally washed away the sediment . It is not known how long it took the river to break through the barrier ; estimates range from several months to several years . Much of the landslide 's debris remained , forcing the river about south of its previous channel and forming the Cascade Rapids . In 1938 , the construction of Bonneville Dam inundated the rapids as well as the remaining trees that could be used to refine the estimated date of the landslide
1700 Cascadia earthquake
1700 Cascadia earthquake
Columbia River