A beaver’s warning sound made by striking the water with its tail is called a tail slap.
Beavers use the slap as an alarm when they detect a possible predator or another danger. The sudden splash can alert nearby family members, which may dive underwater or retreat into the lodge. Because water carries the impact efficiently, the signal can work across a pond or slow stream.
The tail is not simply a noise-making instrument. A beaver’s broad, scaly tail also helps with balance, steering, fat storage, and moving mud or material. Tail slapping is therefore one part of a wider communication system that also includes scent marking, posture, and physical contact.
A common mix-up is to call the sound a “beaver clap.” In wildlife writing, “tail slap” is the standard descriptive term, referring specifically to the action that creates the alarm splash.