New dating of lahar-killed trees underscores volcano hazards in the Puget Sound metropolitan area. Beginning as a landslide from the west flank of Mount Rainier, Washington, the Electron Mudflow swept more than 60 km down the Puyallup River drainage into areas now heavily populated. Wiggle-matching of seven radiocarbon ages from buried, bark-bearing Douglas-fir (Pseudotsuga menziesii) trees brackets the mudflow’s age between 1477–1522 CE with 99.7 percent certainty. The trees with bark all died the same year. Analysis of the Electron chronology using crossdating of tree rings best matched with chronologies from low-elevation sites, especially a Douglas-fir chronology from Vancouver Island, Canada, to show that the Electron trees died in 1507 CE. These data are in three tables, containing properties of the lahar-killed Douglas-fir included in the Electron tree-ring chronology (C14_S1.csv); radiocarbon data from trees killed in the Electron Mudflow (C14_S2.csv); and properties of long tree-ring chronologies in Oregon, Washington, and southern British Columbia and their relationship to the Electron chronology (C14_S3.csv). These are supplementary data tables to support the larger work by Black et al. 2025. Data users are strongly encouraged to refer to that larger work for additional details.