Multiscale structural and lithologic controls in the development of stream potholes on granite bedrock rivers Jose A. Ortega a, ⁎, Miguel Gómez-Heras b , Raúl Perez-López c , Ellen Wohl d a Departamento de Geología y Geoquímica, Universidad Autónoma de Madrid, Madrid 28049, Spain b CEI Campus Moncloa, UPM-UCM, CSIC: ETS Arquitectura, Universidad Politécnica de Madrid and Instituto de Geociencias (CSIC-UCM), Madrid, Spain c IGME, Instituto Geológico y Minero de España, C/Ríos Rosas 23, 28003 Madrid, Spain d Department of Geosciences, Colorado State University, Fort Collins, CO 80523-1482, USA abstract article info Article history: Received 16 April 2013 Received in revised form 6 September 2013 Accepted 10 September 2013 Available online 18 September 2013 Keywords: Potholes Joints Bedrock rivers Spanish Central System Sculpted forms Ultrasonic analysis Jointing, veins, dikes, and fracture patterns influence the genesis of potholes in bedrock rivers. We use measures of surface rock strength (Schmidt hammer readings, joint orientation, and spacing) and subsurface rock proper- ties (ultrasound velocity) to analyze the spatial relationship between joints and potholes in three rivers of the Spanish Central System: the Tietar, Manzanares, and Alberche rivers. At each site, we measured the dimensions of between 45 and 77 potholes; at least 50 randomly located Schmidt hammer rebound values, as well as Schmidt hammer values in a 15 × 15 cm grid around each pothole; and ultrasonic p-wave velocities measured in a 15 × 15 cm grid around a subset of 12 of the potholes. Results support our hypotheses that most potholes corre- late with joints (89% of potholes are related to joint sets), potholes exhibit preferred orientations associated with dominant joints, and pothole genesis correlates more strongly with variations in substrate resistance than with hydraulics. We classify potholes by morphology and present a genetic sequence for specific morphologies in relation to substrate characteristics. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Stream potholes are erosive morphologies usually linked to fluvial processes such as abrasion or hydraulic erosion. Potholes develop in diverse types of substrate, from soft material like clays to resistant bedrock such as granites. Potholes also characterize bedrock fluvial channels with differing levels of incision (Elston, 1917; Alexander, 1932; Lorenc and Saavedra, 1980; Jennings, 1983; Kale and Shingade, 1987; Sato and Hayami, 1987; Wohl, 1993; Hancock et al., 1998; Springer et al., 2006). Several external processes have been described as responsible for pothole genesis, including glacier abrasion (Gilbert, 1906); mechanical abrasion by means of grinders (moulin theory; Charpentier, 1841); depression forms eroded by subglacial meltwater (glaciofluvial theory; Ljunger, 1930); and hydraulic erosion (eddy theory; Alexander, 1932), among others. Whipple et al. (2000) noted that ‘large, often coalescing potholes characterize the lee side of obstructions, protu- berances, and knickpoints’. These authors noted that potholes are related to the position and geometric configuration of the channel, although pothole shape is also related to the ‘formative mechanisms and internal hydraulics’ (Kale and Shingade, 1987; Springer et al., 2005, 2006). Lithology plays an important role in bedrock river erosion from larger scales such as valley geometry and the formation of straths to smaller scales such as the development of sculpted forms (Selby, 1980; Hancock et al., 1998; Richardson and Carling, 2005; Springer et al., 2006; Wohl, 2008). Potholes can form from relatively minor depressions associated with rock heterogeneities that result from weathering or abrasion by boulder impacts (Lorenc et al., 1994; Wang et al., 2009). Joints can be a particularly important form of heterogeneity with respect to pothole initiation and development and can also facilitate block quar- rying (Elston, 1917; Ängeby, 1951; Dubinski and Wohl, 2013). Whipple et al. (2000) highlighted potential interactions between quarrying and potholes: quarrying from zones of more locally jointed rock creates the rough bed and bank topography required to initiate flow separation and vortex formation, which in turn drive abrasion that can lead to potholes; but quarrying can also remove blocks before potholes have sufficient time to form. Springer et al. (2006) demonstrated a strong correlation between potholes and jointing and suggested that joints might create a limit for the depth reached by potholes. Thus, jointing appears to be one of, if not the most important, controls on the occur- rence of potholes. This indicates the need to know more about how the presence of joints, particularly the density and three-dimensional configuration, affects potholes. The erosive mechanisms acting in bedrock rivers are relevant for understanding the beginning of sculpted forms in bedrock rivers, although more in-depth understanding is needed of the processes that result in well-developed potholes. Plucking, abrasion, and cavitation can all contribute to pothole formation (Miller, 1991; Wohl et al., 1994; Hancock et al., 1998; Sklar and Dietrich, 1998; Dubinski and Wohl, 2013). Whipple et al. (2000) established the first-order scaling Geomorphology 204 (2014) 588–598 ⁎ Corresponding author. Tel.: +34 914973833; fax: +34 914974900. E-mail address: j.ortega@uam.es (J.A. Ortega). 0169-555X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.geomorph.2013.09.005 Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph