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Curious Kids: how do currents form under water?

Watch out, currents about. Shutterstock.

Curious Kids is a series by The Conversation, which gives children of all ages the chance to have their questions about the world answered by experts. All questions are welcome: you or an adult can send them – along with your name, age and town or city where you live – to We won’t be able to answer every question, but we’ll do our best.

What forms a current under water? – Natalie, age 11, Melksham, UK.

Thanks for your question, Natalie. Underwater currents can form in lakes, rivers and oceans, and there are many reasons why they happen. Since I’m an ocean scientist, I’m going to explain the currents you find in the sea.

Some ocean currents are very large, and the biggest one – called the “global conveyor belt” – moves water very slowly all the way around the world. In fact, it takes water in the global conveyor belt about 1,000 years to get right around the planet.

Because the global conveyor belt and other big ocean currents move so slowly, we don’t notice them when we go to the beach. But we might feel some other types of currents when we go for a swim.

When ocean waves get to a beach, they turn white at the top and crash onto the sand – this is called “breaking”. Swimming or surfing in breaking waves can be good fun, but we need to remember that these waves cause currents to form.

Wavy days. J_K/Shutterstock.

When waves break on the shore, the sea water in them gets pushed up against the beach. This water must get back out to sea somehow, otherwise we’d expect the water level at the beach to rise and rise forever.

Of course, the water can’t get back out to sea near the surface, because that’s where the breaking waves are busy moving water toward the shore. So, two different currents form, to help take the water back out.

Back out to sea

One of these currents is called the “undertow”. It forms beneath the breaking waves, and pulls the water back toward the sea, across the sandy seabed, out past where the waves are breaking.

Though the undertow helps to get some of the water back to sea, it’s not usually very strong. So, some of the work has to be done by another type of current, called a “rip” current.

Rips are much stronger, narrow currents that run straight out to sea. Rip currents don’t happen all the way along the beach. They only form at certain “weak spots” along the beach where waves are not breaking, and the water is a bit deeper. This makes it easier for the water to flow back out to sea.

Waves break over the sandbars (1), feeder currents form, moving along the shore (2), until meeting and flowing offshore as a rip current (3). Tim Scott., Author provided

Here’s how it works: after water is brought in toward the shore by breaking waves, it can’t turn around and go straight out again, so it runs sideways along the beach in what we call a “feeder current”. As soon as it finds a weak spot, where the waves aren’t breaking, the water flows back out to sea in a rip current.

Staying safe in the surf

It’s very handy to know how to spot a rip current when you go to the beach, because they are much stronger than undertow currents and can sweep people out to sea.

Read more: Rip currents are a natural hazard along coasts – here's how to spot them

When there are lots of waves breaking on the beach, it’s tempting to swim in places where the water looks calmer. But we know that rips form at the places where the waves aren’t breaking – so this is actually the worst place to swim!

Rip currents sometimes leave another tell-tale sign: because they’re so strong, they can churn up the sand on the seabed, making the water look brown and murky.

Even if we know how to spot a rip current, it is always best to swim at beaches where there is a lifeguard, because they’re specially trained to know the best places to swim, and will always be on the look out to make sure everyone is safe.

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