If you’ve taken a walk along a seafront at different times of day, you’ll already be aware of the tides. At high tide, the water comes in close to the shore, and at low tide, it seems far away.
In a lot of places, there will only be a small range, but in other places, the range can be enormous. Read more: Where Are The Highest Tides In The World?
No matter where you are though, the same question arises: where does all the water go at low tide?
At low tide, water moves away from you and towards the “bulge” created by the gravitational effect of the moon and/or the sun. Conversely, when the “bulge” is at your location, water flows towards you, giving you a high tide.
Technically speaking, water does not actually go far at low tide. You can think of it a bit like a blanket on your bed.
When the blanket falls to one side, there is far more of it on one side. To fill in the space that is uncovered, you only need to pull the blanket a short distance.
Each tiny part of the blanket only moves a short distance, yet the overall effect is that the blanket has moved from the side with lots, into the part that was uncovered.
The exact same thing happens with the tides. When the tide flows in or out, it moves towards a “bulge” that is created by the gravitational effect of the moon.
Each molecule of water does not actually move far, but the overall shape of the fluid ocean changes.
When you observe that shape from the relatively small section that you can actually see, it appears as the rising and the falling of the tide.
How water moves during the tides
It is a little hard to explain what happens at low tide without first understanding what causes the tides.
Broadly speaking, the ocean is a body of water that can be pulled and deformed by applying different forces to it.
The big one is the Earth’s gravity. This is the force that stops it flying off into space.
Next, we have the influence of the moon and the sun’s gravity.
The moon exerts a smaller gravitational force than the sun, but it is closer to the Earth. This creates the illusion that they actually have a similar effect.
The sun and the moon both pull at the surface of the oceans, creating a “bulge” in the direction of their pull.
When the sun and the moon pull in the same direction, their effect gets added together, causing a bigger bulge and spring tides.
When they pull at 90 degrees to each other, their effects compete, causing a lower bulge and neap tides.
In addition to the bulge towards the sun and the moon, we also have the effect of the earth’s rotation. You can imagine that the body of water in the diagram is fixed in place, but the earth within it is rotating.
This is why there are two high tides and two low tides per day. The same part of the earth moves through both bulges each day.
Where does the water go at low tide?
We have discussed what causes the tides, but that still doesn’t fully explain where the water goes at low tide.
At low tide, the molecules of water near the beach all move away from the shore a short distance. Equally, molecules of water slightly further out also move away.
The effect is that the entire body of water moves away from the shore at an equal rate.
Overall, the entire body of water, creating the bulges that we see in the diagram above.
When you consider an oceanic body of water, it doesn’t need to bulge upwards by much to cause the tides.
A small bulge across the entire ocean will take a large quantity of water from the shallower edges to form.
This is why tides are most noticeable in shallower waters. They appear to go out a long way when in reality they have only dropped a short distance.
Where does the water come from at high tide?
At high tide, everything works in reverse.
High tides occur when the oceanic bulge of water coincides with your location.
Again, each molecule of water only moves a short distance, but the effect is that the entire oceanic body is dragged in the direction of the forces that are applied by the sun and the moon.
With the sun and the moon pulling the tide towards you, there is nowhere left for the body water to go so it heaps upon the coast.
Currents can be caused by the moving tide
Whenever a body of water moves, we can visualise the movement as a current.
A rising or a falling tide does cause currents as the water slowly moves either towards or away from the coast.
On a wide-open coastline, there would not be a noticeable current because the water will only be moving a few meters over the course of 6 hours.
In a more enclosed area, where a large body of water needs to pass through a relatively small gap, large currents can be created.
If you are trying to drop a large area by only a small amount, there is still a large volume of water to move.
The tidal bulge moves the oceanic body of water outside of the enclosed area, causing a drop in sea level outside. The enclosed area then needs to drop by an equivalent amount so that the water levels match.
When the enclosed area needs to drain through a small entrance, strong currents can form.
The effect of the tide on small pacific islands
Another interesting case when it comes to tides is the effect they have near small islands in the middle of a large body of water.
The Pacific Ocean is a good example because there are lots of small volcanic islands, in the middle of an ocean which covers half the earth.
From these islands, the tide that you experience is a direct result of the bulging of the pacific ocean.
There are no large landmasses for the water to heap up against. It is free to flow around the edges of the relatively small islands.
High tide occurs when the bulge is directly over the island.
After high water, molecules of water slowly spread out. Individually, the molecules do not flow far, but the overall effect is that the bulge of water moves away from the island, across the ocean.