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

Research Article| June 07, 2018 Ongoing bedrock incision of the Fortymile River driven by Pliocene–Pleistocene Yukon River capture, eastern Alaska, USA, and Yukon, Canada Adrian M. Bender; Adrian M. Bender * 1U.S. Geological Survey, 4210 University Drive, Anchorage, Alaska 99508, USA *E-mail: abender@usgs.gov Search for other works by this author on: GSW Google Scholar Richard O. Lease; Richard O. Lease 1U.S. Geological Survey, 4210 University Drive, Anchorage, Alaska 99508, USA Search for other works by this author on: GSW Google Scholar Lee B. Corbett; Lee B. Corbett 2Department of Geology, University of Vermont, 180 Colchester Avenue, Burlington, Vermont 05405, USA Search for other works by this author on: GSW Google Scholar Paul Bierman; Paul Bierman 2Department of Geology, University of Vermont, 180 Colchester Avenue, Burlington, Vermont 05405, USA Search for other works by this author on: GSW Google Scholar Marc W. Caffee Marc W. Caffee 3Department of Physics and Astronomy and Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, 525 Northwestern Avenue, West Lafayette, Indiana 47907, USA Search for other works by this author on: GSW Google Scholar Geology (2018) 46 (7): 635–638. https://doi.org/10.1130/G40203.1 Article history received: 23 Feb 2018 rev-recd: 22 May 2018 accepted: 24 May 2018 first online: 07 Jun 2018 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Adrian M. Bender, Richard O. Lease, Lee B. Corbett, Paul Bierman, Marc W. Caffee; Ongoing bedrock incision of the Fortymile River driven by Pliocene–Pleistocene Yukon River capture, eastern Alaska, USA, and Yukon, Canada. Geology 2018;; 46 (7): 635–638. doi: https://doi.org/10.1130/G40203.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract 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 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 yr. Our observations calibrate a stream-power model (erosion coefficient K ∼ 1.1 × 10–6 m0.2) that closely reproduces the knickzone profile and thus implies long-term (104–106 yr) efficacy of a simple stream-power bedrock incision law. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

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