Two C14 dates on wood from lake sediments that accumulated behind landslide dams in deep canyons of Zion National Park, Utah, permit calculation of the rate of sediment fill. From this, the rate of denudation of the related drainage basin is found to be about 33 inches per 1000 years. Another denudation rate was determined in the headward drainage of Coal Creek, just north of Cedar Breaks National Monument, by the ages of bristle-cone pine trees and the amount of root exposure. This rate is approximately 17 inches per 1000years. Both areas, obviously exposed to rapid erosion, are unusual features in the Colorado River watershed whose average denudation rate is only 6.5 inches per 1000 years (Judson and Ritter, 1964).