Tidal bore, also called bore, body of water that, during exceptionally high sea tides, rushes up some rivers and estuaries near a coast where there is a large tidal range and the incoming tide is confined to a narrow channel. Traveling upstream about two or three times as fast as the normal tidal current, a bore usually is characterized by a well-defined front of one or several waves, often breaking, followed by the bore’s main body, which rises higher than the water level at its front. The height of the bore is greater near the banks of a river than at midstream. Because of momentum, some bores continue to move upstream for about one-half hour after high water. Bores occur at spring tides and at several tides preceding and following spring tides but never at neap tides.