Gezeitenbruch

Gezeitenbruch, also known as tidal bore, is a phenomenon where the leading edge of an incoming tide forms a wave or waves of water that travel up a river or narrow bay against the direction of the river or bay's current. This occurs in relatively few locations around the world where the tide and river/bay characteristics are just right.

The formation of a tidal bore requires a large tidal range, meaning a significant difference between high and low tide, and a funnel-shaped river or bay that narrows inland. As the tide enters the estuary, it is forced into a smaller and smaller space, compressing the incoming water and increasing its height. The shape of the seabed and the riverbed can also contribute to the bore's formation and intensity.

Tidal bores can vary greatly in size and strength. Some are small, gentle ripples, while others can be powerful waves several meters high, capable of traveling many kilometers upstream. The world's most famous and largest tidal bore is the Qiantang River bore in China. Other notable examples occur on the Amazon River in Brazil, the Severn River in the United Kingdom, and the Bay of Fundy in Canada.

These bores can be a significant attraction for tourists and surfers, but they also pose a hazard to riverine navigation and can cause flooding. The timing of a tidal bore is predictable, tied to the lunar cycle and the tidal patterns of the region.