Rates of river incision and scarp retreat in eastern and central Grand Canyon over the past half million years: Evidence for passage of a transient knickzone: COMMENT

Abbott et al. (2015) and Crow et al. (2014), two Grand Canyon incision studies, come to very different conclusions despite apparent methodological similarities. Crow et al. (2014) used U-Th, Ar-Ar, and cosmogenic burial dating of material associated with Colorado River (CR) strath terrace sequences at 6 sites throughout Grand Canyon and concluded that average bedrock incision rates have been temporally steady at each site over at least the last 650 ka, but vary spatially from 100 to 160 m/Ma due to differential mantle-driven uplift. Abbott et al. (2015) used our unpublished Ar-Ar dating on a dike in western Grand Canyon, near river mile 159, plus U-Th dating of travertine-cement in sidestream alluvium, near Hermit Rapid, to suggest that incision rates were 1–4 km/Ma from 500 to 400 ka and <200 m/Ma after 400 ka, a difference they ascribe to a migrating knickpoint. The conclusions of these studies are contradictory.