bore

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.

bore Very rapid rise of the tide, in which the advancing flood waters form a wave with an abrupt front. Bores occur in certain shallow estuaries and river mouths where there is a large tidal range, and suitably funnel-shaped regions, e.g. the Amazon, the Bay of Fundy, the Tsing Kiang River in China, and the Rivers Severn, Trent, and Ouse in England.

bore, or eager, a sudden and rapid flow of the tide in certain rivers and estuaries which rolls up in the form of a wave. Bores are caused either by the meeting of two tides, where the excess of water results in a rapid rise, or by a tide rushing up a narrowing estuary where the closeness of the banks or a shelving bottom encloses the tide so that it is forced to rise rapidly to accommodate the water coming in.