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.

tidal bore
This is what an actual tidal wave looks like.

Title Wave
Famous tidal bores have unique names.

  • aegir (Trent River, England)
  • benak (Batang River, Malaysia)
  • mascaret (Seine River, France)
  • pororoca (Amazon River, Brazil)
  • silver dragon (Qiantang River, China)
benak
Noun

local term for tidal bore, or tidal wave, especially the tidal bore of the Batang Lupar (or Lupar River) near Sri Aman, Malaysia.

canal
Noun

artificial waterway.

cargo
Noun

goods carried by a ship, plane, or other vehicle.

Noun

organism that eats meat.

churn
Verb

to mix vigorously or violently.

Noun

edge of land along the sea or other large body of water.

Noun

steady, predictable flow of fluid within a larger body of that fluid.

dam
Noun

structure built across a river or other waterway to control the flow of water.

Noun

the flat, low-lying plain that sometimes forms at the mouth of a river from deposits of sediments.

dredge
Verb

to remove sand, silt, or other material from the bottom of a body of water.

Noun

branch of biology that studies the relationship between living organisms and their environment.

Noun

mouth of a river where the river's current meets the sea's tide.

Noun

any system for capturing, storing, checking, and displaying data related to positions on the Earth's surface.

high tide
Noun

water level that has risen as a result of the moon's gravitational pull on the Earth.

Noun

watering land, usually for agriculture, by artificial means.

Noun

body of land surrounded by water.

mascaret
Noun

French term for tidal bore, or tidal wave, such as the tidal bore of the Seine River.

Noun

place where a river empties its water. Usually rivers enter another body of water at their mouths.

neap tide
Noun

the lowest level of high tide when the difference between low and high tide is the least, occurring when the gravitational pull of the sun counteracts that of the moon.

negative surge
Noun

a sudden shallowing of a channel's depth.

new moon
Noun

dark phase of the lunar cycle when the moon is invisible or barely visible, occurring when the moon passes between the sun and earth.

Noun

large body of salt water that covers most of the Earth.

pavilion
Noun

a low-lying, open, semi-permanent shelter.

pororoca
Noun

local term for tidal bore, or tidal wave, especially of the Amazon River.

positive surge
Noun

deepening of a water channel.

river management
Noun

the art and science of controlling the flow, path, and power of rivers.

sandbar
Noun

mound of sand created by water currents.

Noun

organism that eats dead or rotting biomass, such as animal flesh or plant material.

Noun

solid material transported and deposited by water, ice, and wind.

shipping
Noun

transportation of goods, usually by large boat.

Noun

small sediment particles.

spring tide
Noun

tide occuring during the times of full and new moon that "springs" to above-average highs and lows.

surge
noun, verb

sudden, strong movement forward.

Noun

tidal wave. Tide flowing upstream against the current of a river, forming a wave of water.

tidal range
Noun

the difference in height between an area's high tide and low tide.

vegetation
Noun

all the plant life of a specific place.