TREE-RING DATING OF THE BONNEVILLE LANDSLIDE TO LATE 1446 OR EARLY 1447 CE

Wiggle-match radiocarbon dating and tree-ring analyses on three subfossil trees from Columbia River Gorge in the Pacific Northwest yield a provisional date for the Bonneville landslide of late 1446 or early 1447 CE during dormancy. The Bonneville landslide was a large (>15.5 km²) rockslide-debris avalanche from Table Mountain in Columbia River Gorge that filled the valley bottom and impounded a temporary lake as much as 75-m deep. Rising water killed trees along the former floodplain of the Columbia at least 63 km upstream of the landslide dam, described as the “submerged forest of the Columbia” in journals of early explorers. Two of the submerged forest trees we studied were sampled in 1934 near the shore at Wyeth and at Perham Creek, Oregon by botanist Donald Lawrence. The third analyzed tree is a bark-bearing log exhumed from the landslide deposit during excavations for Bonneville Dam’s second powerhouse in 1978. We cross correlated tree rings of each of the subfossil trees, all Douglas-firs (Pseudotsuga menziesii), with the others by visual techniques and by analyzing 0.01-mm-resolution growth ring measurements using programs COFECHA and CDendro. Results show all three trees died the same year. Radiocarbon wiggle-matching of nine ages from the three trees indicates tree death, and hence the Bonneville landslide, occurred within a 2σ range of 1426–1448 CE.

We obtained a more precise date for the landslide by cross dating the three victim trees to regional tree-ring chronologies using programs ARSTAN and CDendro. A final annual ring of 1446 CE for the Columbia Gorge victim tree chronology correlates with an old growth Douglas-fir located ~2000 ft elevation 5.5 km northwest of Bonneville Dam, with a master chronology at Big Lava Beds, Washington, and with two unpublished chronologies at Soapgrass Mountain and Canyon Creek, Oregon. The Gorge chronology also cross dates with a chronology of 14 trees in the Puyallup River valley, 133 km north of the Gorge, that were buried by the Electron Mudflow from Mount Rainier and cross dated to late 1507. A trigger for the Bonneville landslide has yet to be identified, but this precise date will help assess possible relationships with other events, assist with interpreting Indigenous narratives about the Bonneville landslide, and educate the region about potential geologic hazards.