Multiscale structural and lithologic controls in the development of stream potholes on granite bedrock rivers

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 properties (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 correlate 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.