Undercuts (rocks and banks) usually are well-known hazards on a river and often are the site of fatalities.
- River banks, especially outside bends, are constantly eroded by moving water. The bank becomes undercut, creating an entrapment. Trees also can topple into the water from the undermining and create strainers.
- One well-known example of this is Dimple Rock on the Youghiogheny River at Ohiopyle State Park in Pennsylvania.