Stopper

From Wikipaddle

Stoppers are formed when water pours over the top of a submerged object, causing the surface water to flow back upstream toward the object. Holes can be particularly dangerous, a boater may become stuck in the recirculating water. Playboaters use the holes' features to perform various playboating moves. In high-volume water, holes dramatically aerate the water, possibly to the point where it may even lose the capacity to carry any water crafts.

Some of the most dangerous types of holes are formed by wiers, underwater ledges, and similar types of obstruction. In weirs, the hole has a very symmetrical character there's no weak point and the sides of the stopper are often blocked by a man-made wall, making it impossible to slip off the side of the stopper.