Tidal Bore, a wall of water that travels upstream from the mouth of a tidal river as the tide rises. (A tidal river is one that empties into a body of water that has pronounced tides. The tides in the larger body cause the level of the river to rise and fall.) Bores occur only in a few rivers. The largest bore in the world occurs in the Tsientang Kiang (Qiantang Jiang), a river in China, where an 11-foot (3.4-m) wall of water travels upstream at the rate of more than 17 miles per hour (27 km/h). In North America, a tidal bore occurs in the Petitcodiac River at the head of the Bay of Fundy in Canada. Rivers at the head of the Gulf of California and at the head of Cook Inlet, Alaska, also have bores. The Amazon in South America has a notable bore.
The formation of a tidal bore requires a precise balance of conditions. There must be a great rise in water level between low tide and high tide. The river must decrease rapidly in depth upstream from its mouth. In addition, the river channel must become narrower upstream. Only when these conditions exist in the necessary relationship will a bore form. A bore can be destroyed by changing the relationship, such as by dredging a river to deepen it. Bores occur shortly after low tide. Although the bore is the most spectacular part of the rising tide, the rise of water continues after the passage of the bore until high tide is reached.