USE OF DENDROCHRONOLOGY TO DATE AND BETTER UNDERSTAND THE BONNEVILLE LANDSLIDE, COLUMBIA RIVER GORGE, WASHINGTON

The Bonneville landslide in the Columbia River Gorge is significant to the geologic, environmental, and cultural history of the Pacific Northwest. Native peoples memorialized this landslide in allegories about “the Bridge of the Gods”. Lewis and Clark noted the landslide and described the “submerged forest” that had been drowned by the lake it formed. Age estimates for the landslide ranged from A.D. 1400 to 1750 at the beginning of this study. We have described, mapped, and cored more than 40 old-growth trees living on the landslide deposit and have used dendrochronology to evaluate this forest refugia and estimate its minimum age. The oldest tree sampled thus far began growing before A.D. 1595. Therefore, considering likely ecesis time, the Bonneville landslide probably dates to before A.D. 1550. Many of the oldest trees on the Bonneville landslide are situated on “balds” consisting of rubbly Columbia River Basalt Group masses that were rafted in the landslide and now support little or no soil. Thus, the intensity and damage of past fires (as evidenced by the extent and age of the Douglas fir refugia) has been somewhat limited by the geological aspects of the landslide deposit. In our ongoing research, we are sampling trees along the margins of the landslide and its headscarp to look for those that may have been damaged or otherwise affected by the landslide. These trees could provide additional clues to the age of the landslide event. We will also look more closely at the geology of the landslide as well as attempt to find old-growth trees along its margins that may predate it and could record movement of the landslide in their tree rings. We have discovered a submerged forest in Crescent Lake, on the adjacent Red Bluffs landslide, that may provide more clues to the age of that landslide. This research was greatly assisted by a Partners in Science grant from the Murdock Charitable Trust.