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body of water that, during exceptionally high sea tides, rushes up some rivers. 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. Not arising in estuaries, tidal bores are formed at a position a short distance upstream, where the river channel has become sufficiently narrow or shallow to concentrate the momentum of the rising tide. Bores occur at spring tides and at several tides preceding and following spring tides but never at neap tides. The formidable tidal bore that occurs on the lower Seine in France between Rouen and the sea is known as the Mascaret.