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.
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)
Term Part of Speech Definition Encyclopedic Entry 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.
goods carried by a ship, plane, or other vehicle.
organism that eats meat.
Encyclopedic Entry: carnivore churn Verb
to mix vigorously or violently.
edge of land along the sea or other large body of water.
Encyclopedic Entry: coast current Noun
steady, predictable flow of fluid within a larger body of that fluid.
Encyclopedic Entry: current dam Noun
structure built across a river or other waterway to control the flow of water.
the flat, low-lying plain that sometimes forms at the mouth of a river from deposits of sediments.
Encyclopedic Entry: delta dredge Verb
to remove sand, silt, or other material from the bottom of a body of water.
branch of biology that studies the relationship between living organisms and their environment.
Encyclopedic Entry: ecology estuary Noun
mouth of a river where the river's current meets the sea's tide.
Encyclopedic Entry: estuary geographic information system (GIS) Noun
any system for capturing, storing, checking, and displaying data related to positions on the Earth's surface.
Encyclopedic Entry: GIS (geographic information system) high tide Noun
water level that has risen as a result of the moon's gravitational pull on the Earth.
watering land, usually for agriculture, by artificial means.
Encyclopedic Entry: irrigation island Noun
body of land surrounded by water.
Encyclopedic Entry: island mascaret Noun
French term for tidal bore, or tidal wave, such as the tidal bore of the Seine River.
place where a river empties its water. Usually rivers enter another body of water at their mouths.
Encyclopedic Entry: mouth 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.
large body of salt water that covers most of the Earth.
Encyclopedic Entry: ocean pavilion Noun
a low-lying, open, semi-permanent shelter.
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.
mound of sand created by water currents.
organism that eats dead or rotting biomass, such as animal flesh or plant material.
Encyclopedic Entry: scavenger sediment Noun
solid material transported and deposited by water, ice, and wind.
Encyclopedic Entry: sediment shipping Noun
transportation of goods, usually by large boat.
small sediment particles.
Encyclopedic Entry: silt 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.
tidal bore Noun
tidal wave. Tide flowing upstream against the current of a river, forming a wave of water.
Encyclopedic Entry: tidal bore tidal range Noun
the difference in height between an area's high tide and low tide.
all the plant life of a specific place.