Distinguishing between tectonic and lithologic controls on bedrock channel longitudinal profiles using cosmogenic 10 Be erosion rates and channel steepness index Andrew J. Cyr a, ⁎, Darryl E. Granger a , Valerio Olivetti b , Paola Molin b a Department of Earth and Atmospheric Sciences, Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47907, USA b Dipartimento di Scienze Geologiche, Università degli Studi “Roma Tre”, Largo San Leonardo Murialdo 1, 00146 Roma, Italy abstract article info Article history: Received 8 November 2010 Received in revised form 21 November 2013 Accepted 1 December 2013 Available online xxxx Keywords: Erosion Channel longitudinal profiles Cosmogenic nuclides Channel steepness Italy Knickpoints in fluvial channel longitudinal profiles and channel steepness index values derived from digital ele- vation data can be used to detect tectonic structures and infer spatial patterns of uplift. However, changes in lith- ologic resistance to channel incision can also influence the morphology of longitudinal profiles. We compare the spatial patterns of both channel steepness index and cosmogenic 10 Be-determined erosion rates from four land- scapes in Italy, where the geology and tectonics are well constrained, to four theoretical predictions of channel morphologies, which can be interpreted as the result of primarily tectonic or lithologic controls. These data indi- cate that longitudinal profile forms controlled by unsteady or nonuniform tectonics can be distinguished from those controlled by nonuniform lithologic resistance. In each landscape the distribution of channel steepness index and erosion rates is consistent with model predictions and demonstrates that cosmogenic nuclide methods can be applied to distinguish between these two controlling factors. Published by Elsevier B.V. 1. Introduction Conceptual (Gilbert, 1877) and mathematical (Howard, 1994; Howard et al., 1994; Whipple and Tucker, 1999, 2002) models of rivers undergoing steady and uniform tectonic and climatic forcing predict smooth, concave-up channel longitudinal profiles. Deviations from this predicted longitudinal profile form, e.g., convexities or knickpoints, are commonly cited as evidence for disequilibrium conditions (here, the term knickpoint is used to denote any abrupt change of channel gradi- ent between adjacent channel reaches that are evident in a longitudinal profile). Knickpoints are often attributed to either nonuniform (in space) or unsteady (in time) surface uplift associated with either active or ancient tectonic structures. However, knickpoints can also be formed when a river encounters a relative change in the erodibility of its sub- strate (Hack and Hack, 1957; Hack, 1973) because, in order to continue incising at a rate similar to its upstream and downstream sections, a channel segment flowing over more resistant rocks must be steeper. Descriptions of the dynamics of river incision (Whipple and Tucker, 1999, 2002), in combination with the availability of digital elevation data from which river longitudinal profiles can be extracted, have pro- vided new analytic techniques for quantifying different morphologic aspects of river longitudinal profiles, such as channel steepness and con- cavity (Snyder et al., 2000; Kirby and Whipple, 2001). These techniques have provided a powerful tool for studying landscape evolution (Ouimet et al., 2009; Olivetti et al., 2012) and active tectonics (Kirby et al., 2003; Wobus et al., 2006; Kirby et al., 2007; Wilson et al., 2009). However, the distinction between a knickpoint generated by tectonic forcing and one generated by changes in lithologic resistance to channel incision may be difficult to make from digital elevation data alone. The question then becomes, when a knickpoint is observed in a bedrock channel longitudinal profile without any a priori knowledge of the drainage basin, what does that knickpoint represent? 1.1. Theoretical predictions Given several simplifying assumptions, bedrock river incision rate, E, can be expressed as a function of shear stress (Howard and Kerby, 1983; Sklar and Dietrich, 1998), E ¼ KA m S n ð1Þ where A is the upstream drainage area (a proxy for discharge); S is the local channel slope; m and n are positive constants that depend on basin hydrology, hydraulic geometry, and incision process; and K is a dimensional coefficient of erosion with units of m 1–2m /y and depends on a variety of factors, including rock strength, the amount and size of channel bed material, debris flow frequency, and the drainage area– discharge and channel length–channel width relationships (Howard et al., 1994; Sklar and Dietrich, 1998; Stock and Montgomery, 1999; Whipple and Tucker, 1999; Snyder et al., 2000; Whipple and Tucker, 2002). K remains poorly calibrated, with evidence showing that it can vary over several orders of magnitude between different study areas (Stock and Montgomery, 1999). Geomorphology xxx (2013) xxx–xxx ⁎ Corresponding author at: U.S. Geological Survey, 345 Middlefield Road MS973, Menlo Park, California 94025, USA. Tel.: +1 650 329 4820; fax: +1 650 329 4936. E-mail address: acyr@usgs.gov (A.J. Cyr). GEOMOR-04585; No of Pages 12 0169-555X/$ – see front matter. Published by Elsevier B.V. http://dx.doi.org/10.1016/j.geomorph.2013.12.010 Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph Please cite this article as: Cyr, A.J., et al., Distinguishing between tectonic and lithologic controls on bedrock channel longitudinal profiles using cosmogenic 10 Be erosion rates and channel steepness index, Geomorphology (2013), http://dx.doi.org/10.1016/j.geomorph.2013.12.010