The timing of Missoula floods: Implications for the age of Grand Coulee (eastern Washington, USA)

The Channeled Scabland of eastern Washington (USA) was formed by outburst floods from glacial Lake Missoula. Despite chronological advances, the timing of erosion in the main flood channels is unresolved. In particular, it is still uncertain whether upper Grand Coulee, the largest canyon in the Channeled Scabland, was incised during or prior to the last glaciation. We report 10Be exposure ages from erratics in upper Grand Coulee, glacial Lake Columbia, and surrounding flood routes. Flood-transported boulders on the high-elevation east rim of Grand Coulee date to ca. 17−15 ka. Ages from boulders on the floor of Grand Coulee indicate later flooding at ca. 14 ka, which post-dated canyon incision and occurred after inundation of the Telford-Crab Creek scabland at ca. 15−14.5 ka. Prior hydraulic modeling and dating suggest the entrance to Grand Coulee was blocked by rock and that canyon incision was incomplete at ca. 17 ka; hence, we interpret the 17−15 ka exposure ages on the east rim to coincide with flow over a retreating cataract during canyon incision. Our results indicate incision of Grand Coulee was completed between 17 ka and 14 ka. The short duration of canyon incision suggests that glacial Lake Missoula generated some of the most erosive outburst floods in Earth’s history.