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

Crow et al. (2014) and Abbott et al. (2015) were the first studies to use river terrace age data to extend the incision record for eastern and central Grand Canyon beyond ca. 400 ka. An accurate understanding of Grand Canyon’s long-term incision history has obvious geomorphic significance, but it also has geodynamic importance in light of recently proposed models for late Cenozoic, mantle-driven surface uplift on the Colorado Plateau (e.g., Moucha et al., 2008; 2009; Roy et al., 2009; Levander et al., 2011; Roberts et al., 2012; Karlstrom et al., 2012; Becker et al., 2014). Crow et al. (2014) interpreted their incision results as strong support for the Karlstrom et al. (2007; 2008) model of temporally steady but spatially variable Grand Canyon incision since at least 3–4 Ma, which they attributed to mantle-induced surface uplift. By contrast, Abbott et al. (2015) concluded, based on data from two locations, that the incision rate in eastern and central Grand Canyon was rapid between ca. 500–400 ka during passage of a transient knickzone and subsequently decreased to the rate measured by previous workers (Crow et al., 2014; Pederson et al., 2002; 2006; Karlstrom et al., 2007). In their Comment, Crow et al. (2015a) raised four objections to our conclusions: (1) Our study did not constrain the ancient elevation of the Colorado River at either location; (2) Our U/Th ages at the Hermit Creek study site (river mile [RM] 96) do not record the depositional age of the strath terrace; (3) Our transient knickzone model is incompatible with seven “well-dated” river gravels presented by Crow et al. (2014); (4) The ~340 m-high knickpoint implied by our model is “unreasonable” and our proposed incision rate during knickzone passage is 2–8 times greater than the highest known knickpoint-controlled bedrock incision rate in the southwestern U.S. We discuss each of these criticisms in turn.