Loosely, a vertical cave system; more precisely, a more or less circular hole in the bedrock of a river. The hole enlarges because pebbles inside it collide with the bedrock as the water swirls. Potholes carved into streambeds can be important components of channel incision. Springer et al. (2005) Geomorph. 82, 1–2 demonstrate that the pothole dimensions of radius and depth are strongly correlated.