A tidal bore, often simply given as bore in context, is a tidal phenomenon in which the leading edge of the incoming tide forms a wave of water that travels up a river or narrow bay, reversing the direction of the river or bay's current. It is a strong tide that pushes up the river, against the current.
A bore in Morecambe Bay, in the United Kingdom
The tidal bore in Upper Cook Inlet, in Alaska
Undular bore and whelps near the mouth of Araguari River in northeastern Brazil. The view is oblique towards the mouth from airplane at approximately 30 m (100 ft) altitude.
The Trent Aegir seen from West Stockwith, Nottinghamshire, 20 September 2005
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and are also caused by the Earth and Moon orbiting one another.
In Maine (U.S.), low tide occurs roughly at moonrise and high tide with a high Moon, corresponding to the simple gravity model of two tidal bulges; at most places however, the Moon and tides have a phase shift.
Low tide at Bangchuidao scenic area, Dalian, Liaoning Province, China
Low tide at Ocean Beach in San Francisco, California, U.S.
Low tide at Bar Harbor, Maine, U.S. (2014)