The regular rise and fall of the ocean’s waters are known as tides. Along coasts, the water slowly rises up over the shore and then slowly falls back again. When the water has risen to its highest level, covering much of the shore, it is at high tide. When the water falls to its lowest level, it is at low tide. Some lakes and rivers can also have tides.

Causes of Tides


Forces that contribute to tides are called tidal constituents. The Earth’s rotation is a tidal constituent. The major tidal constituent is the moon’s gravitational pull on the Earth. The closer objects are, the greater the gravitational force is between them. Although the sun and moon both exert gravitational force on the Earth, the moon’s pull is stronger because the moon is much closer to the Earth than the sun is.

The moon’s ability to raise tides on the Earth is an example of a tidal force. The moon exerts a tidal force on the whole planet. This has little effect on Earth’s land surfaces, because they are less flexible. Land surfaces do move, however, up to 55 centimeters (22 inches) a day. These movements are called terrestrial tides. Terrestrial tides can change an object’s precise location. Terrestrial tides are important for radio astronomy and calculating coordinates on a global positioning system (GPS). Volcanologists study terrestrial tides because this movement in the Earth’s crust can sometimes trigger a volcanic eruption.

The moon’s tidal force has a much greater effect on the surface of the ocean, of course. Water is liquid and can respond to gravity more dramatically.

High Tides
The tidal force exerted by the moon is strongest on the side of the Earth facing the moon. It is weakest on the side of the Earth facing the opposite direction. These differences in gravitational force allow the ocean to bulge outward in two places at the same time. One bulge occurs on the side of the Earth facing the moon. This is the moon’s direct tidal force pulling the ocean toward it. The other bulge occurs on the opposite side of the Earth. Here, the ocean bulges in the opposite direction of the moon, not toward it. The bulge may be understood as the moon’s tidal force pulling the planet (not the ocean) toward it.

These bulges in the ocean waters are known as high tides. The high tide on the side of the Earth facing the moon is called the high high tide. The high tide caused by the bulge on the opposite side of the Earth is called the low high tide. In the open ocean, the water bulges out toward the moon. Along the seashore, the water rises and spreads onto the land.

Low Tides and Ebb Tides
One high tide always faces the moon, while the other faces away from it. Between these high tides are areas of lower water levels—low tides. The flow of water from high tide to low tide is called an ebb tide.


Most tides are semidiurnal, which means they take place twice a day. For example, when an area covered by the ocean faces the moon, the moon’s gravitational force on the water causes a high high tide. As the Earth rotates, that area moves away from the moon’s influence and the tide ebbs. Now it is low tide in that area. As the Earth keeps rotating, another high tide occurs in the same area when it is on the side of the Earth opposite the moon (low high tide). The Earth continues spinning, the tide ebbs, another low tide occurs, and the cycle (24 hours long) begins again.

The vertical difference between high and low tide is called the tidal range. Each month, the range changes in a regular pattern as a result of the sun’s gravitational force on the Earth. Although the sun is almost 390 times farther away from the Earth than is the moon, its high mass still affects the tides.

Because the Earth’s surface is not uniform, tides do not follow the same patterns in all places. The shape of a seacoast and the shape of the ocean floor both make a difference in the range and frequency of the tides. Along a smooth, wide beach, the water can spread over a large area. The tidal range may be a few centimeters. In a confined area, such as a narrow, rocky inlet or bay, the tidal range could be many meters. The lowest tides are found in enclosed seas like the Mediterranean or the Baltic. They rise about 30 centimeters (about a foot). The largest tidal range is found in the Bay of Fundy, Canada. There, the tides rise and fall almost 17 meters (56 feet).

Twice each month, the moon lines up with the Earth and sun. These are called the new moon and the full moon. When the moon is between the Earth and the sun, it is in the sun’s shadow and appears dark. This is the new moon. When the Earth is between the sun and moon, the moon reflects sunlight. This is the full moon.

When the sun, moon and Earth are all lined up, the sun’s tidal force works with the moon’s tidal force. The combined pull can cause the highest and lowest tides, called spring tides. Spring tides happen whenever there is a new moon or a full moon and have nothing to do with the season of spring. (The term comes from the German word springen, which means “to jump.”)

In the period between the two spring tides, the moon faces the Earth at a right angle to the sun. When this happens, the pull of the sun and the moon are weak. This causes tides that are lower than usual. These tides are known as neap tides.

Tidal Features


Tides produce some interesting features in the ocean. Tides are also associated with features that have nothing to do with them.

A tidal bore occurs along a coast where a river empties into the ocean or sea. The tidal bore is a strong tide that pushes up the river, against the river's current. This is a true tidal wave. The huge tidal bore of the Amazon River is called the pororoca. The pororoca is a wave up to 4 meters (13  feet) tall, traveling at speeds of 15 kilometers (9 miles) per hour. The pororoca travels 10 kilometers (6 miles) up the Amazon.


While a tidal bore is a tidal wave, a tsunami is not. Tsunami is taken from the Japanese words for “harbor wave.” Tsunamis are caused not by tides, but by underwater earthquakes and volcanoes. Tsunamis are associated with tides because their reach surpasses the tidal range of an area.

So-called “red tides” also have nothing to do with actual tides. A red tide is another term for an algal bloom. Algae are microscopic sea creatures. When billions of red algae form, or “bloom,” in the ocean, the waves and tides appear red.

Finally, rip tides are not a tidal feature. Rip tides are strong ocean currents running along the surface of the water. A rip tide runs from the shore back to the open ocean. Rip tides can be helpful to surfers, who use them to avoid having to paddle out to sea. Rip tides can also be very dangerous to swimmers, who can be swept out to sea.

Intertidal Life

The land in the tidal range is called the intertidal zone. The intertidal zone is often marked by tide pools. Tide pools are areas that are completely underwater at high tide but remain as pockets of seawater when the tide ebbs. Tide pools are home to some of the ocean’s richest biodiversity.

The intertidal zone can be hard-bottomed or soft-bottomed. A zone with a hard bottom is rocky. A zone with a soft bottom has silt or sand. Wetlands and marshes are often soft-bottomed intertidal zones. Different creatures have adapted to different types of intertidal zones. Hard-bottom zones often have barnacles and seaweeds, while soft-bottom zones have more sea plants and slow-moving creatures like rays.

Intertidal zones are marked by vertical zonation. Different organisms live in different zones in the tidal range, depending on how much water reaches them. This zonation can often be seen vertically, with dry plants near the top of the tidal zone and seaweeds near the bottom.

The intertidal zone can be broken into four major mini-zones. The highest is called the splash zone (1). This area is splashed by water and mist during high tide, but is never fully underwater. Barnacles live on rocks in the splash zone. Many marine mammals, such as seals and sea otters, can live in the splash zone.

The high-tide zone (2) is pounded by strong waves. Animals that live in the high-tide zone often have strong shells and are able to cling tightly to rocks to avoid being swept out to sea. These animals include mussels and barnacles. Crabs, which have tough exoskeletons and can hide under rocks, also live in the high-tide zone.

The mid-tide zone (3) is usually the busiest part of the intertidal zone. This is where tide pools usually form. Animals from the high- and low-tide zones come here to feed. Animals that live in the mid-tide zone are still tough, but can have softer bodies than their neighbors in the high-tide zone. Brightly colored sea anemones, which are soft-bodied but strongly anchored to rocks, live in tide pools. Snails and hermit crabs use shells to protect their soft bodies.


Sea stars (sometimes called starfish, although they are not related to fish at all) are perfectly adapted to life in tide pools. They have a tough, leathery body that can withstand strong tides and waves. They have thousands of tiny, tube-like legs that help them stick to rocks or put them on the move for prey. Sea stars are carnivores, and will eat anything, such as fish, snails, or crabs. They especially love mussels. The way sea stars eat is unusual. Sea stars move over a mussel and use their arms to pry open the mussel’s shell. Then, the sea star ejects its own stomach to surround the mussel. The sea star’s stomach contains powerful acids that dissolve the mussel and make it easy to digest when the sea star pulls its stomach back into its body.

The low-tide zone (4) is only dry at the lowest tide. Nudibranchs, a type of sea slug, live in tide pools in the low-tide zone. Like the sea star, this animal is a carnivore. Nudibranchs eat sponges, barnacles and other nudibranchs. Nudibranchs can also eat sea anemones, because they are immune to its poisonous tentacles.

People can be very active in the low-tide zone. Simple nets can catch fish here, and fishers can collect animals like crabs, mussels, and clams. “The tide is out, our table is set,” is a traditional saying among the Tlingit nation (tribe), who live along the Pacific Northwest coast in Alaska and Canada.

In the low-tide zone of the Puget Sound in the U.S. state of Washington, people practice tidal aquaculture. Aquaculture is the breeding, raising, and harvesting of plants and animals that live in the water. One of the most harvested animals is a giant clam called a geoduck. Geoduck farms have been set up in the Puget Sound tidelands, which are areas covered by the intertidal zone. On the farms, geoducks live in plastic pipes. Environmental groups worry about the impact of these pipes on the environment. Tools of aquaculture, such as unsecured pipes, nets, and rubber bands, can be washed away by tides. This debris can pollute the ocean, beach, and natural tide pools.

Tides and People


Tidal energy is a renewable resource that many engineers and consumers hope will be developed on a large scale. Now, small programs in Northern Ireland, South Korea, and the U.S. state of Maine are experimenting with harnessing the power of tides.

There are three different types of tidal power. All of these use tidal energy generators to convert that power into electricity for use in homes and industry.

In most tidal energy generators, turbines are put in tidal streams (1). A turbine is a machine that takes energy from a flow of fluid. That fluid can be air (wind) or liquid (water). Because water is more dense than air, tidal energy is more powerful than wind energy. Placing turbines in tidal streams can be difficult, because the machine disrupts the tide it is trying to harness. However, once the turbines are in place, tidal energy is predictable and stable.

Another tidal energy generator uses a type of dam called a barrage (2). A barrage is a low dam where water can spill over the top or through turbines in the dam. Barrages can be constructed across tidal rivers and estuaries. Turbines inside the barrage can harness the power of tides the same way a dam can harness the power of a river. Barrages are more complex designs than single turbines.

The final type of tidal energy generator is a tidal lagoon (3). The lagoons function much like barrages, but are usually constructed out of more natural materials, like rocks. Tidal lagoons can sit along coasts and do not prevent the natural migration of wildlife.

Geographic imaging systems (GIS) rely on tidal calculations. GIS must account for tides when mapping, especially when mapping the ocean floor. Tides affect the report on an area’s depth.

Predicting tides is very important for shipping and travel across oceans. Ships decide which channels they may navigate by calculating their own weight, the depth of the ocean and an area’s tidal range. Errors in navigation can strand ships along shores or on sand banks. Cargo can sit and spoil while waiting for a tide. This was not a significant problem after the 2004 tsunami in Southeast Asia. Even though the tsunami destroyed kilometers of coastline, GIS technology helped disaster-relief agencies get aid to victims in Indonesia, Thailand, and Sri Lanka.

Cause and Effect: Tides
Raking in the clams.

Surfing the Dragon
In rivers with strong tidal bores, surfing is a popular recreational sport. The worlds strongest tidal bore is on the Qiantang River in southern China. This tidal wave can be 9 meters (30 feet) high and travel at 40 kilometers per hour (25 miles per hour). Surfers rarely remain upright for more than 10 seconds. Athletes call surfing the Qiantang surfing the dragon.

Watch Out
Tidal flatsthe low-lying areas that are underwater at high tide and dry at low tidecan be dangerous places. In soft-bottomed intertidal zones off Alaskas Pacific shore, for instance, the mud is several feet thick. People have wandered out onto the tidal flats, gotten stuck in the mud and drowned when the tide rushed in.

A Really High Tide
The same gravitational force that creates a high tide can create a black hole. The moons tidal force pulls in the Earths ocean, creating a tide. At the right distance, a black holes tidal force pulls in everything in its pathincluding light. (And once youre in a black hole, there is no low tide!)

acid
Noun

chemical compound that reacts with a base to form a salt. Acids can corrode some natural materials. Acids have pH levels lower than 7.

adapt
Verb

to adjust to new surroundings or a new situation.

algae
Plural Noun

(singular: alga) diverse group of aquatic organisms, the largest of which are seaweeds.

algal bloom
Noun

the rapid increase of algae in an aquatic environment.

anchor
Verb

to hold firmly in place.

aquaculture
Noun

the art and science of cultivating marine or freshwater life for food and industry.

barrage
Noun

a low dam.

Noun

body of water partially surrounded by land, usually with a wide mouth to a larger body of water.

Noun

narrow strip of land that lies along a body of water.

Noun

all the different kinds of living organisms within a given area.

canal
Noun

artificial waterway.

cargo
Noun

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

Noun

organism that eats meat.

celestial
Adjective

having to do with the sky or heavens.

Noun

waterway between two relatively close land masses.

Noun

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

complex
Adjective

complicated.

construct
Verb

to build or erect.

consumer
Noun

person who uses a good or service.

coordinates
Noun

a set of numbers giving the precise location of a point, often its latitude and longitude.

Noun

rocky outermost layer of Earth or other planet.

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.

debris
Noun

remains of something broken or destroyed; waste, or garbage.

dense
Adjective

having parts or molecules that are packed closely together.

digest
Verb

to convert food into nutrients that can be absorbed.

disrupt
Verb

to interrupt.

dissolve
Verb

to break up or disintegrate.

dramatic
Adjective

very expressive or emotional.

earthquake
Noun

the sudden shaking of Earth's crust caused by the release of energy along fault lines or from volcanic activity.

ebb tide
Noun

tide that flows from high tide to low tide.

eject
Verb

to get rid of or throw out.

electricity
Noun

set of physical phenomena associated with the presence and flow of electric charge.

engineer
Noun

person who plans the building of things, such as structures (construction engineer) or substances (chemical engineer).

Noun

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

exert
Verb

to force or pressure.

exoskeleton
Noun

the hard external shell or covering of some animals.

fluid
Noun

material that is able to flow and change shape.

geoduck
Noun

type of large, burrowing clam.

Noun

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

Global Positioning System (GPS)
Noun

system of satellites and receiving devices used to determine the location of something on Earth.

gravitational pull
Noun

physical attraction between two massive objects.

gravity
Noun

physical force by which objects attract, or pull toward, each other.

hard-bottomed
Adjective

shore or coast with a rocky lower layer.

harness
Verb

to control or guide for a specific purpose.

hermit crab
Noun

type of marine animal (crustacean) that uses found materials, such as other creatures' shells, as its shell.

high high tide
Noun

tide created when the Earth directly faces the moon.

high tide
Noun

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

high-tide zone
Noun

coastal area that is partially submerged at high tide.

industry
Noun

activity that produces goods and services.

inlet
Noun

small indentation in a shoreline.

Noun

region between the high and low tide of an area.

Noun

body of water surrounded by land.

liquid
Noun

state of matter with no fixed shape and molecules that remain loosely bound with each other.

Noun

position of a particular point on the surface of the Earth.

low high tide
Noun

tide created when the Earth faces away from the moon.

low tide
Noun

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

low-tide zone
Noun

mostly submerged coastal area that is only exposed at low tide.

mapping
Noun

making and using maps.

marine mammal
Noun

an animal that lives most of its life in the ocean but breathes air and gives birth to live young, such as whales and seals.

Noun

wetland area usually covered by a shallow layer of seawater or freshwater.

mass
Noun

unit of measurement (abbreviated m) determined by an object's resistance to change in the speed or direction of motion.

microscopic
Adjective

very small.

mid-tide zone
Noun

coastal area that is underwater at high tide, partially submerged as the tide ebbs, and exposed at low tide.

Noun

movement of a group of people or animals from one place to another.

Noun

clouds at ground-level, but with greater visibility than fog.

Noun

natural satellite of a planet.

navigate
Verb

to plan and direct the course of a journey.

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.

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.

nudibranch
Noun

brightly colored marine organism (gastropod), also called a sea slug.

Noun

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

Noun

large, spherical celestial body that regularly rotates around a star.

poisonous
Adjective

toxic or containing dangerous chemicals.

pollute
Verb

to introduce harmful materials into a natural environment.

pororoca
Noun

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

precise
Adjective

exact.

predator
Noun

animal that hunts other animals for food.

predict
Verb

to know the outcome of a situation in advance.

prey
Noun

animal that is hunted and eaten by other animals.

radio astronomy
Noun

the study of outer space using radio waves.

ray
Noun

flat-bodied fish with fins that appear to flap like wings.

red tide
Noun

the rapid, dense accumulation of algae or other plankton that contain red or brown pigments; also called algal bloom.

renewable resource
Noun

resource that can replenish itself at a similar rate to its use by people.

rip tide
Noun

strong current that runs along the surface of the ocean from shore to open sea. Also called rip current.

Noun

large stream of flowing fresh water.

Noun

object's complete turn around its own axis.

sand
Noun

small, loose grains of disintegrated rocks.

sand bank
Noun

large underwater deposit of sand, often tall enough to reach the water's surface.

Noun

large part of the ocean enclosed or partly enclosed by land.

sea anemone
Noun

type of marine animal related to corals and jellies.

sea star
Noun

marine animal (echinoderm) with many arms radiating from its body. Also called a starfish.

seaweed
Noun

marine algae. Seaweed can be composed of brown, green, or red algae, as well as "blue-green algae," which is actually bacteria.

semidiurnal
Adjective

occuring twice a day.

shell
Noun

hard outer covering of an animal.

shipping
Noun

transportation of goods, usually by large boat.

shore
Noun

coast.

significant
Adjective

important or impressive.

Noun

small sediment particles.

soft-bottomed
Adjective

shore or coast with a soft lower layer of silt or sand.

splash zone
Noun

coastal area above the high-tide zone that is only submerged during storms.

spoil
Verb

to rot or ruin.

spring tide
Noun

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

stable
Adjective

steady and reliable.

starfish
Noun

sea star. Marine animal with multiple arms that can cling to rocks or move about. Sea stars are not fish.

stomach
Noun

organ in animals that helps digest food.

Noun

the sport of riding down a breaking wave on a board.

surpass
Verb

to go beyond a set limit.

technology
Noun

the science of using tools and complex machines to make human life easier or more profitable.

tentacle
Noun

a long, narrow, flexible body part extending from the bodies of some animals.

terrestrial tide
Noun

the effect of the moon's tidal force on land surfaces of the Earth.

Noun

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

tidal constituent
Noun

force that helps create a tide.

Noun

energy produced as ocean waters surge in and out with tides.

tidal energy generator
Noun

machine for turning tidal energy into electricity humans can use.

tidal flat
Noun

coastal wetlands, often found within the intertidal zone, formed when mud is deposited by tides.

tidal force
Noun

gravitational pull exerted by one object, such as the sun or moon, that raises tides on another object, such as the Earth.

tidal lagoon
Noun

pool of ocean water that is partially cut off from the ocean by a barrier. Often used as a source of hydroelectric power.

tidal range
Noun

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

tidal stream
Noun

an ocean current produced by the tide.

Noun

rise and fall of the ocean's waters, caused by the gravitational pull of the moon and sun.

tidelands
Noun

intertidal zone. Region between the high tide and the low tide of an area.

tide pool
Noun

small pond created by an ebb tide and submerged by a high tide.

tribe
Noun

community made of one or several family groups sharing a common culture.

tsunami
Noun

ocean waves triggered by an earthquake, volcano, or other movement of the ocean floor.

turbine
Noun

machine that captures the energy of a moving fluid, such as air or water.

uniform
Adjective

exactly the same in some way.

vertical zonation
Noun

the arrangement of species adapted to different, layered areas of a larger ecosystem.

volcanic eruption
Noun

activity that includes a discharge of gas, ash, or lava from a volcano.

Noun

an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.

volcanologist
Noun

scientist who studies volcanoes.

wave
Noun

moving swell on the surface of water.

Noun

area of land covered by shallow water or saturated by water.

wildlife
Noun

organisms living in a natural environment.

Noun

movement of air (from a high pressure zone to a low pressure zone) caused by the uneven heating of the Earth by the sun.

Noun

kinetic energy produced by the movement of air, able to be converted to mechanical power.