Richard O. Lease Lee B. Corbett Paul R. Bierman Marc Caffee Adrian Bender 2018 <div id="114752102" class="article-section-wrapper js-article-section js-content-section "><p>Quantification of river incision via process rate laws represents a key goal of geomorphic research, but such models often fail to reproduce traits of natural rivers responding to base-level lowering. The Fortymile River flows from eastern Alaska in the United States to the Yukon River in Canada across a tectonically quiescent region with near-uniform precipitation and bedrock erosivity. We exploit these stable boundary conditions to quantify bedrock incision evident in a gravel-capped strath terrace that flanks the lower ∼175 km of the river and grades to the minimally incised headwaters. The terrace gravel yields a cosmogenic isochron burial age of 2.44 ± 0.24 Ma, consistent with abandonment triggered by late Pliocene–early Pleistocene Yukon River headwater capture. The deeply incised reach forms a linear knickzone where basin relief nearly doubles and inferred bedrock incision rates (∼19–110 m/m.y.) averaged since ca. 2.44 Ma increase downstream toward the Fortymile–Yukon River confluence. Basin-scale<span>&nbsp;</span>¹⁰Be-based erosion rates of tributaries to the Fortymile River trunk nearly double from the headwaters (∼9 mm/k.y.) to the knickzone (average ∼16 mm/k.y.), revealing the pace of ongoing landscape response to knickzone incision over 10⁴<span>&nbsp;</span>yr. Our observations calibrate a stream-power model (erosion coefficient<span>&nbsp;</span><i>K</i><span>&nbsp;</span>∼ 1.1 × 10^–6<span>&nbsp;</span>m^0.2) that closely reproduces the knickzone profile and thus implies long-term (10⁴–10⁶<span>&nbsp;</span>yr) efficacy of a simple stream-power bedrock incision law.</p></div> application/pdf 10.1130/G40203.1 en Geological Society of America Ongoing bedrock incision of the Fortymile River driven by Pliocene–Pleistocene Yukon River capture, eastern Alaska, USA, and Yukon, Canada article