When a tidal wave enters into the channel of a river, the sea water presents obstruction in its smooth flow. On the other hand, the river water, if it is in larger volume, presents hurdle in the progress of the tidal wave upstream.

Thus, in the shallow and narrow channel of the river, the tidal wave gains abnormal height. The front part of such a tidal wave which has grown in height stands as a wall, while the back water assumes an elongated shape. These tidal waves are, therefore, called a tidal bore.

Due to these high waves, the direction of flow of the river is reversed to a certain distance. With the further movement of the tidal bore upstream, there is a continuous decrease in its height due to friction with the bottom of the river channel until it finally dies out.

The great difference in elevation between the sea level in the est­uary mouth and the floor of the estuary higher up causes the water to break into a wave with foaming crest, which rushes up the estuary very rapidly. The height of the bore varies in accordance with the tide in the bay.

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The bore occurs at many places in estuaries on the coast. Among the places, where the bore is produced, are the Severn in England, the Seine in France, the Amazon, and some of the rivers in China. The Petitcodiac bore, New Brunswick is well-known for its great regularity. The bore is given different names in different localities.

The bore is known as mascarat in the Seine; in the Amazon it is called powcoa or pororoca. In India, the tidal bore occurs in the river Hooghly. The best example of a tidal bore in the world is that (tidal bore) on the Tsien-tang-kiang River in northern China, where the front of the wave is about 3 meters high.

The rate of its upstream advance is 16 km an hour. The bore in the Trent River in England is locally called eager. In this river the bore moves upstream at the speed of 24 km an hour and its height is about 2 meters. It may be noted that the maximum development of a bore occurs at the time of spring tide.