A dip in the surface of a river
followed by a recirculating wave
. Caused by water rushing over a shallow rock or ledge.
There are two basic kinds of holes: 'smiling' holes and 'frowning' holes. The labels reference the shape of the wave at the downstream edge of the hole as viewed from upstream. 'Smiling' holes will tend to throw objects downstream at their edges and 'frowning' holes will tend to keep object forever.
Holes are popular play spots for whitewater paddlers. The can be surfed, spun, squirted, and are the place where whitewater rodeo takes place. Someone who spends a long time in a hole surfing and pulling other moves while others are waiting is likely to be labelled a 'hole hog'.