What is a standing wave that breaks back upstream after flowing over an obstruction referred to as?

A standing wave that breaks back upstream after flowing over an obstruction is referred to as a hole. This phenomenon occurs when water flows over an obstacle, such as a rock or a ledge, creating turbulent water that appears to be "standing" in one place while the current flows underneath it. As water flows over the obstruction, it can create a depression or an area of low pressure, resulting in the water being drawn back toward the obstruction, thereby forming the characteristic appearance of a hole.

This behavior is essential for water rescue technicians to understand, as holes can be very dangerous for paddlers and swimmers due to the forceful nature of the water and the potential for being trapped. Recognizing the characteristics of a hole is crucial for navigation and safety in water rescue scenarios.