A tidal bore is a large, turbulent, wall-like wave of water that moves inland or upriver as an incoming tidal current surges against the flow of a more narrow and shallow river, bay, or estuary. It can be 10 to 16 feet (3 to 5 meters) high and move rapidly (10 to 15 knots) upstream with, and faster than, the rising tide.

What are rip tides and why are they so dangerous?

At points along a coast where waves are high, a substantial buildup of water is created near the shore. This mass of water moves along the shore until it reaches an area of lower waves. At this point, it may burst through the low waves and move out from shore as a strong surface current moving at an abnormally rapid speed known as a rip current. Swimmers who become exhausted in a rip current may drown unless they swim parallel to the shore.