bore, inrush of water that advances upstream with a wavelike front, caused by the progress of incoming tide from a wide-mouthed bay into its narrower portion. The tidal movement tends to be retarded by friction as it reaches the shallower water and meets the river current; it therefore piles up and forms a low wall of water that moves upstream with considerable force and velocity as the tide continues to rise. In the mouth of the Amazon River a tidal bore known locally as the pororoca occurs every spring tide. It has a wall of water from 5 to 15 ft (1.5–4.6 m) high and advances at a speed of from 10 to 15 mi (16–24 km) per hr. The highest recorded bore (15 ft/4.6 m) is found in the Fuchun River near Hangzhou, China. Bores are found also in the Bay of Fundy, in Solway Firth, in the Severn, Seine, and Hugli rivers, and in Hangzhou Bay.

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