Ongoing bedrock incision of the Fortymile River driven by Pliocene-Pleistocene Yukon River capture, eastern Alaska, USA, and Yukon, Canada

摘要

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 similar to 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 knick-zone where basin relief nearly doubles and inferred bedrock incision rates (similar to 19-110 m/m.y.) averaged since ca. 2.44 Ma increase downstream toward the Fortymile-Yukon River confluence. Basin-scale Be-10-based erosion rates of tributaries to the Fortymile River trunk nearly double from the headwaters (similar to 9 mm/k.y.) to the knickzone (average similar to 16 mm/k.y.), revealing the pace of ongoing landscape response to knickzone incision over 10(4) yr. Our observations calibrate a stream-power model (erosion coefficient K similar to 1.1 x 10(-6) m(0.2)) that closely reproduces the knickzone profile and thus implies long-term (10(4)-10(6) yr) efficacy of a simple stream-power bedrock incision law.