Ongoing bedrock incision of the Fortymile River driven by Pliocene–Pleistocene Yukon River capture, eastern Alaska, USA, and Yukon, Canada
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.
Citation Information
| Publication Year | 2018 |
|---|---|
| Title | Ongoing bedrock incision of the Fortymile River driven by Pliocene–Pleistocene Yukon River capture, eastern Alaska, USA, and Yukon, Canada |
| DOI | 10.1130/G40203.1 |
| Authors | Adrian Bender, Richard O. Lease, Lee B. Corbett, Paul R. Bierman, Marc Caffee |
| Publication Type | Article |
| Publication Subtype | Journal Article |
| Series Title | Geology |
| Index ID | 70237325 |
| Record Source | USGS Publications Warehouse |
| USGS Organization | Alaska Science Center Geology Minerals |
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