craft to fall through the aerated water to the bottom of a deep 'hole'.

Some of the most dangerous types of holes are formed by low-head dams (weirs), and similar types of obstructions. In a low-head dam, the 'hole' has a very wide, uniform structure with no escape point, and the sides of the hydraulic (ends of the dam) are often blocked by a man-made wall, making paddling around, or slipping off, the side of the hydraulic, where the bypass water flow would become normal (laminar), difficult. By (upside-down)