Field measurements of incision rates following bedrock exposure: Implications for process controls on the long profiles of valleys cut by rivers and debris flows

Research Article| January 01, 2005 Field measurements of incision rates following bedrock exposure: Implications for process controls on the long profiles of valleys cut by rivers and debris flows Jonathan D. Stock; Jonathan D. Stock 1Department of Earth and Planetary Science, University of California, Berkeley, California 94720, USA Search for other works by this author on: GSW Google Scholar David R. Montgomery; David R. Montgomery 2Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195, USA Search for other works by this author on: GSW Google Scholar Brian D. Collins; Brian D. Collins 2Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195, USA Search for other works by this author on: GSW Google Scholar William E. Dietrich; William E. Dietrich 3Department of Earth and Planetary Science, University of California, Berkeley, California 94720, USA Search for other works by this author on: GSW Google Scholar Leonard Sklar Leonard Sklar 3Department of Earth and Planetary Science, University of California, Berkeley, California 94720, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Jonathan D. Stock 1Department of Earth and Planetary Science, University of California, Berkeley, California 94720, USA David R. Montgomery 2Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195, USA Brian D. Collins 2Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195, USA William E. Dietrich 3Department of Earth and Planetary Science, University of California, Berkeley, California 94720, USA Leonard Sklar 3Department of Earth and Planetary Science, University of California, Berkeley, California 94720, USA Publisher: Geological Society of America Received: 09 Jan 2004 Revision Received: 06 May 2004 Accepted: 10 May 2004 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2005) 117 (1-2): 174–194. https://doi.org/10.1130/B25560.1 Article history Received: 09 Jan 2004 Revision Received: 06 May 2004 Accepted: 10 May 2004 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Jonathan D. Stock, David R. Montgomery, Brian D. Collins, William E. Dietrich, Leonard Sklar; Field measurements of incision rates following bedrock exposure: Implications for process controls on the long profiles of valleys cut by rivers and debris flows. GSA Bulletin 2005;; 117 (1-2): 174–194. doi: https://doi.org/10.1130/B25560.1 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 SocietyGSA Bulletin Search Advanced Search Abstract Until recently, published rates of incision of bedrock valleys came from indirect dating of incised surfaces. A small but growing literature based on direct measurement reports short-term bedrock lowering at geologically unsustainable rates. We report observations of bedrock lowering from erosion pins monitored over 1–7 yr in 10 valleys that cut indurated volcanic and sedimentary rocks in Washington, Oregon, California, and Taiwan. Most of these channels have historically been stripped of sediment. Their bedrock is exposed to bed-load abrasion, plucking, and seasonal wetting and drying that comminutes hard, intact rock into plates or equant fragments that are removed by higher flows. Consequent incision rates are proportional to the square of rock tensile strength, in agreement with experimental results of others. Measured rates up to centimeters per year far exceed regional long-term erosion-rate estimates, even for apparently minor sediment-transport rates. Cultural artifacts on adjoining strath terraces in Washington and Taiwan indicate at least several decades of lowering at these extreme rates. Lacking sediment cover, lithologies at these sites lower at rates that far exceed long-term rock-uplift rates. This rate disparity makes it unlikely that the long profiles of these rivers are directly adjusted to either bedrock hardness or rock-uplift rate in the manner predicted by the stream power law, despite the observation that their profiles are well fit by power-law plots of drainage area vs. slope. We hypothesize that the threshold of motion of a thin sediment mantle, rather than bedrock hardness or rock-uplift rate, controls channel slope in weak bedrock lithologies with tensile strengths below ∼3–5 MPa. To illustrate this hypothesis and to provide an alternative interpretation for power-law plots of area vs. slope, we combine Shields' threshold transport concept with measured hydraulic relationships and downstream fining rates. In contrast to fluvial reaches, none of the hundreds of erosion pins we installed in steep valleys recently scoured to bedrock by debris flows indicate any postevent fluvial lowering. These results are consistent with episodic debris flows as the primary agent of bedrock lowering in the steepest parts of the channel network above ∼0.03–0.10 slope. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.