A tidal bore occurs along a coast where a river empties into an ocean or sea. A tidal bore is a strong tide that pushes up the river, against the current. A tidal bore is a true tidal wave.
A tidal bore is a surge. A surge is a sudden change in depth. When a channel suddenly gets deeper, it experiences a positive surge. When a channel suddenly gets shallower, it experiences a negative surge. Tidal bores are positive surges.
Not all coasts feature tidal bores. In fact, there are few places where tidal bores occur. The river must be fairly shallow. It must have a narrow outlet to the sea. However, the estuary, or place where the river meets the sea, must be wide and flat. The coast’s tidal range—the area between high tide and low tide—must be quite large, usually at least 6 meters (about 20 feet). When all of these conditions are met, a tidal bore is formed.
There are exceptions. The Amazon River is the largest river in the world. It empties into the Atlantic Ocean. The mouth of the Amazon is not narrow, but the river still has a strong tidal bore. A tidal bore develops here because the mouth of the river is shallow and dotted by many low-lying islands and sand bars. The tidal bore, called the pororoca, is so strong that the Amazon does not have a delta. Its sediment is emptied directly into the Atlantic and carried away by fast-moving currents.
Tides are stable and can be predicted. Tidal bores are less predictable. The development of tidal bores depends on a number of factors, including wind and the depth of the river. A change in a river’s depth can be affected by rainfall or shipping traffic. Tidal bores can occur every day, like the tidal bore of the Batang River in Malaysia, called the benak. Other tidal bores, like the pororoca, occur during spring tides. Spring tides happen during new moons and full moons, when tides are strongest. Tidal bores almost never occur during neap tides. Neap tides happen during quarter moons, when tides are weakest.
Despite some unpredictability, few observers are surprised by tidal bores. Along the Qiantang River in Hangzhou, China, site of the world’s largest tidal bore, observers gather at tide-watching pavilions to observe the 9-meter (30-foot) wave. The roar of the tidal wave can be heard for hours before it bores up the river.
The leading edge of the Qiantang River tidal bore can move as fast as 40 kilometers (25 miles) per hour. The tide behind the wave makes the river's water rise for hours after the bore passes.
A tidal bore can be quite violent. The bore often changes the color of the river from blue or green to brown as it whips up sediment. Tidal bores can tear vegetation like trees from their roots. This makes the recreation sports of river surfing and kayaking very dangerous. Surfers from China to Alaska have been pulled into the river, bay or ocean. Even watching a bore can be dangerous: Tidal waves have been known to sweep over lookout points and drag people to the churning river.
Tidal bores have a direct impact on the ecology of the river mouth. Animals slammed by the leading edge of a tidal wave can be left dazed or dead in the silty water. For this reason, carnivores and scavengers are common sights behind tidal bores. In the Amazon, piranhas gobble up fish, crabs, and even birds left behind by the wave. Crocodiles swim behind the Styx River bore in Queensland, Australia. The Cook Inlet in the U.S. state of Alaska experiences strong tidal bores. Bears and eagles wade into the water hours after the wave passes to pick up fish along the banks.
Human activity can change or even remove tidal bores. A century ago, the Seine River in France had a strong tidal bore, called the mascaret. Years of river management (canals, dams, irrigation systems, dredging) eliminated the mascaret. Before the French began managing the Seine, the unpredictable mascaret was responsible for the loss of hundreds of ships. The wave would rush up the river, upsetting cargo ships and destroying docks.
This threat to shipping is still a problem in areas with tidal bores. Navigators rely on sophisticated instruments, including geographic information system (GIS) technology. They also rely on knowledge of the ocean, the river and the riverbed to calculate the size and strength of tidal bores.