INTERTIDAL EVIDENCE FOR A COLUMBIA RIVER DAM-BREAK FLOOD IN THE 15TH CENTURY CE

An anomalous unit of gray silt and clay exposed at low tide provides stratigraphic evidence for rapid incision of the 15th century Bonneville Landslide, which dammed the Columbia River some 50 km upstream of Portland, Oregon. First described in the early 1990s, this probable flood deposit is being reexamined to refine estimates of its age, to check for signs of coeval seismic shaking, and to provide ground truth for flood simulations. The unit is distinctively paired with an overlying waterlaid tephra previously correlated geochemically with the 1479 Wn eruption of Mount St. Helens. Plant detritus within and beneath the unit provides material for radiocarbon dating. The unit’s total thickness ranges from 1 to 8 cm. Its internal structure varies among sites, with some outcrops displaying two or three fining-upward layers while others contain just one. Sand dikes evidence ground shaking in some outcrops, though those observed thus far erupted at stratigraphic levels decimeters below and above the unit but not at the level of the probable flood deposit itself. Study of this downstream deposit joins other efforts to understand the timing, cause, and impact of the Bonneville landslide.