What Really Causes Tides and Why the Answer Is Stranger Than You Think

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When Laura Dekker was 11 years old, she anchored her small keelboat in the Wadden Sea and went to sleep. She had selected a place that appeared deep enough. What she neglected to consider was that the sea had no intention of remaining there.

“I woke up at night because my small keelboat had fallen over in the soft mud as the tide had run out,” the Dutch resident told ZME Science. “Oops! I remember it very well. It was a good lesson.”

This is one of the ocean’s more effective teaching lessons. It doesn’t issue a warning or let you retake the exam. It simply removes the water and leaves your bedroom lying at an angle.

At just 14, Dekker set off aboard a 40-foot (12-meter) ketch named Guppy. About 17 months later, she had completed a solo circumnavigation with stops and became widely regarded as the youngest person to do so. Guinness had quit recognizing the age-based record, and the World Sailing Speed Record Council does not recognize “youngest” sailing records, but the achievement itself is not disputed.

As someone who would eventually cross the Indian, Pacific and Atlantic oceans, Dekker could still be caught out by a patch of disappearing water. Tide charts are predictable enough to print, but even the most seasoned sailors can still tip their boats.

The Moon is not working alone

Most of us receive a simple explanation for tides in school: The Moon’s gravity pulls on the ocean, creating a bulge of water and therefore a high tide.

The simplified explanation is useful but incomplete. The Moon and Sun supply astronomical forcing. Earth’s rotation, continents, seafloor topography and friction determine how the oceans respond. Weather and river flow then modify the water levels and currents people encounter along the coast.

“The tide is kind of a wave, but it’s really a forced disturbance on the ocean surface,” Merrick Haller, an Oregon State University professor who studies hazardous waves and coastal currents, told ZME Science. “There are dozens of components that cause part of the tidal signal.”

There is also the puzzle of why many coasts experience two high tides instead of one. If the Moon merely pulled water toward itself, there should be a single bulge facing it. Instead, another forms on the opposite side of Earth.

The reason is that the Moon pulls on the entire planet. It attracts the ocean, the rocks beneath it, the mantle, the iron core, the continents, your house and every cup of coffee. But it does not pull on all of them equally.

Its pull is slightly stronger on the nearest side and slightly weaker on the far side. Relative to Earth’s center, this uneven force stretches the planet and its oceans along the Earth-Moon line, creating the tendency for two bulges.

The Sun generates tides as well. However, although its total gravitational attraction on Earth is far stronger, it is also much farther away. Tidal force depends on how much gravity changes across Earth, and the Sun’s tide-generating influence is about half the Moon’s.

When the Sun, Earth and Moon align during a new or full moon, their effects reinforce one another, generally producing higher high tides and lower low tides. These are called spring tides which occur throughout the year.

The problem with continents

The familiar double-bulge model works best on an imaginary planet covered by water of equal depth. This world has no Africa, no English Channel and nowhere for anyone to park a boat, but it makes the equations pleasantly tidy.

Actual oceans are broken into basins of different shapes and depths. Continents block the water, the seafloor slows it and Earth’s rotation curves its movement. The tide behaves as an enormous, forced wave that reflects from coastlines and crosses continental shelves.

“The complicating factor is that water has to move,” Haller said. “There’s friction against the Earth, and that makes the simple bulge model imperfect. The topography and bathymetry of the ocean basins really affect how that bulge is able to propagate in or out of a given area.”

In some basins, the crest of a particular tidal component rotates around an amphidromic point, where that component produces little change in water level. Elsewhere, reflected waves reinforce one another and create much larger tides. The local response explains why some coasts receive two similar high tides each lunar day, others receive one and still others experience two highs of very different sizes.

Canada’s Bay of Fundy is the perfect example of tides. Near the head of the bay, the difference between high and low water can reach about 52 feet (16 meters), roughly the height of a four-story building.

Every partly enclosed body of water has one or more natural periods at which it prefers to slosh. Water in the Bay of Fundy takes about 13 hours to rock from the mouth of the bay to its head and back. The Atlantic tide arrives every 12 hours and 25 minutes, repeatedly pushing the bay near its preferred rhythm.

“If you shake it at the frequency it prefers, its resonant frequency, then it sloshes out,” Haller said, using his water glass as an example. “If you oscillate at a completely different frequency, it just sloshes a bit.”

Fundy also narrows and becomes shallower toward its head, concentrating the incoming water. Resonance and geography together produce Earth’s highest tides.

When a general incoming tide enters a shallow, narrowing river or estuary, it can steepen into a tidal bore—a moving surge sometimes capped with whitewater.

Unlike most large waves, tidal bores keep approximate appointments. Spectators gather to watch, and surfers in some places ride them for miles.

When the tide starts moving sideways

There is an important difference between a tidal wave and a tidal current, says Haller. The tidal wave is the immense pattern of changing water level moving through an ocean basin. A tidal current is the local horizontal flow as water enters or leaves a bay, strait or harbor.

In the open ocean, a vessel may barely notice the surface tide. Near a coast or narrow passage, the horizontal current can become impossible to ignore. At the mouth of the Columbia River, the largest river in the U.S.’s Pacific Northwest region, Haller said tidal currents can reach several feet per second.

Field measurements in Chile’s Chacao Channel have recorded tidal currents exceeding 13 feet (four meters) per second, or about nine miles per hour. That is not fast for a car, but it is exceptionally fast when the moving object is an enormous body of water.

Currents become dangerous when water is squeezed through a constriction or when an ebb tide combines with river discharge. Wind and waves moving against the current can bunch together, becoming steeper and more likely to break. Harbor pilots need accurate forecasts because a passage that is manageable at one stage of the tide may become hazardous an hour later.

Dekker encountered some of the strongest and least straightforward tidal flows of her life around the Faroe Islands. She had already grown accustomed to tides around the Netherlands, North Sea, English Channel and Wadden Sea.

“Careful planning is crucial,” Dekker said. “In the Faroe Islands I learned to appreciate (tides). I liked planning it well and then going at great speed to the next destination.”

Currents don’t always feel dramatic aboard a boat because the vessel and surrounding water move together. Sailors often notice it through changes in their speed over the seabed or patterns at the surface.

“I can see it on the water,” Dekker said. “Depending on the wind strength and direction, the water can either be very smooth or incredibly choppy. It’s often possible to see ripples and swirls with strong tidal flows.”

Dekker learned the theory from her father, but experience taught her how that theory looks in real water.

“Careful planning is always necessary, but I learned to see and read it by simply observing a lot, feeling my surroundings and talking to others with more experience,” she said.

Charts provide tide and current predictions, but local sailors may know where the water accelerates or an eddy forms.

“Ask around,” Dekker said, who now operates the Laura Dekker World Sailing Foundation, an organization to teach youth how to sail.

Sailors arriving at the wrong time usually means waiting, not disaster. However, not all is lost if you arrive late (or early) to the party.

“It just means you won’t be going anywhere for a few hours,” Dekker said.

An ocean beneath the ocean

Some of the most dramatic tidal motions are hidden below the surface. Fresher or warmer water tends to rest above saltier or colder water because it is less dense. The boundary between these layers can carry waves like a submerged ocean surface.

When tidal currents cross an underwater ridge or shelf edge, they can push that boundary up and down, generating internal tides. Haller and his colleagues documented a related phenomenon at the Columbia River, where fresh river water rests above denser seawater. As the ebb tide moves seaward, the boundary can form an internal hydraulic jump.

“What’s cool about the Columbia River one is this is not at the surface,” Haller said. “It’s at the boundary between the fresh water and the salt water.”

The jump produces a detectable surface signature that researchers can follow with remote-sensing instruments. Similar tidal interactions with underwater ridges generate immense internal waves in the South China Sea. These waves alter currents and the depth of density layers, creating conditions important to submarine navigation and underwater acoustics.

This is a magnificently understated way of saying that the calm-looking ocean may be doing something quite forceful underneath.

Learning not to argue with the ocean

Laura Dekker in 2019. Photo: Stephan Redel

Strong tidal flows can prevent a sailor from leaving harbor or stop a boat short of shelter after days at sea. Dekker has nevertheless come to view them as opportunities as well as obstacles.

“Sailing isn’t about going places fast,” she said. “It’s about appreciating and working with nature. Once you’re able to realize that, it’s easier to let that frustration go and see the beauty of it.”

A carefully timed current can carry a boat swiftly through a channel without requiring more wind or fuel. The same force that had appeared to be an obstacle becomes a means of travel.

“There is something satisfying in being able to use nature to go from A to B,” Dekker said. “It’s amazing what the power of the Moon does to our waters. The ever-shifting winds and tides are mesmerizing to watch when you live with them day and night—simply becoming part of it.”

This does not mean she has achieved uninterrupted peace with the ocean.

“Don’t get me wrong, I still yell at the waves, and sometimes I still get frustrated,” Dekker said. “But the most valuable lessons sailing taught me are patience, appreciation and a deep, deep respect for nature.”

The Moon begins the tide, but it does not complete it. The Sun modifies it, Earth’s rotation turns it, continents block it, ocean basins redirect it and bays occasionally magnify it into something extravagant. By the time all those forces reach a small anchored boat, the result is local, complicated and perfectly capable of depositing an 11-year-old sailor sideways in the mud.