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What is an Undertow?

Mary McMahon
Updated Jun 04, 2024
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An undertow is a type of ocean current that is caused by waves breaking on the shore. Most are quite mild and not dangerous, as long as swimmers keep their heads, although some have been known to be powerful enough to sweep swimmers out to sea. Incidentally, an undertow is not the same thing as a rip current, although the two currents are both caused by breaking waves on the shore.

To understand how an this current works, a person can think about what happens when waves break on shore. The water obviously has to go somewhere, and this is what causes an undertow: as waves break, water from previous waves runs underneath them, creating a gentle current that runs back out to sea. When there is heavy wave action, the water may not be able to get out and as a result, it builds up and seeks a weak point in the breaking waves. When the water finds a weak point, it pushes out to sea, creating a rip current.

The difference between these two types of currents is significant. Most undertows are not very strong, and the risk of one is most severe for inexperienced swimmers who are standing or swimming near breaking waves. An undertow can pull someone underwater for a few seconds, but if the swimmer remains calm and swims towards the surface, he or she should be OK. This current is not usually strong enough to prevent the swimmer from returning to shore, unlike a rip current, which could carry the swimmer out to sea.

When swimmers encounter strong undertows, the tempting thing to do is to push towards the shore in the hopes of breaking through the current. This is actually a terrible idea, as swimmers can tire themselves out before they reach the shoreline. The best thing to do is to swim parallel to the shore, testing for a weak point that will allow the swimmer to get back to shore; the same technique works for rip currents. If a swimmer tires, he or she should tread water and float in the hopes that a rescuer will arrive soon.

Whenever someone visits an area with unfamiliar waters, it is a good idea for them to ask about prevailing currents. Locals can alert swimmers about areas with especially strong undertows, or areas where rip currents often arise. As a general rule, the steeper the slope that the waves approach on, the stronger the resulting current; long shallow approaches create much less wave velocity.

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Mary McMahon
By Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a All Things Nature researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Discussion Comments
By anon1003565 — On Jul 24, 2020

I love the comments here. Undertow is a rip current, and if anyone has any knowledge about beaches, they understand that undertow does cause deaths to the unwary in high sea conditions.

Simply put, rip currents and undertow are the means by which the water caused by the current from waves is cleared from a shoreline. Undertow is frequently found on steep stony beaches where the water surge in a break has no other escape route. If someone is caught in a high undertow, the person may lose their footing and can be pulled into a towering break.

A rip is normally fully visible due to the feature of lower waves that have smaller break areas, so with a level head, a swimmer caught in one can swim across to the eddy of two crossing currents and escape the rush, not so with a strong undertow, as the eddy is often under the surface.

In rescue service and surfing, riding a rip and training in practical rescue and self rescue is great to do. Sit on a surf beach and watch a local surfer use the rip like a conveyor. It can be a great tool. It takes knowledge and a cool head, but self rescue is highly possible.

Undertow is more dangerous when the victim is dragged under in that they could tumble in the current, hit the seabed and bounce around, which may knock the breath out of someone. It’s the toe testers who often get caught and lose a life, as the current pulls them from the shore.

Now waves at sea are just energy carriers, rarely doing anything other than re-directing unpowered vessels unless storm lifted. Waves come to land under the surface, so the surface tension means the water on top rides up and over the energy flow. Once the energy gets to a shallowing shoreline, the surface stretches, eventually breaking into the loose broken water we all know as surf.

This whole thing is driven by the sun's actions, where all storms start. This explains why some surf has no power, or why after big ocean storms, the rings (like a stone splash in a pond echos rings) run for land carrying levels of energy. Fear and respect the sea. It’s dangerous even to those who work with it.

Respect the water; it's indifferent toward you!

By anon261125 — On Apr 14, 2012

Why is everybody trying to split hairs over this? Who gives a rip what it's called? The underlying and most important information was given about how to survive such an event.

It's unfortunate about the victims who have suffered this phenomenon, which brings to light the importance of education about strange or new swimming areas and never swimming alone. Life is life, and surviving these currents no matter what you want to call them, is the most important.

By anon189228 — On Jun 22, 2011

nice thing to tell a 10 year old. I am an adult and the story you just told is sad. Very, very sad. If it bothers me, imagine how it affected a child.

By anon188521 — On Jun 21, 2011

You guys don't have it right. You're all talking about rip currents, which take you out to sea. That's what killed that unfortunate man. Not an undertow. Undertows do not pull you under, sort of. An undertow is basically spinning you around like you're in a washing machine. The dangerous undertows form where there is a steep shore, sandy especially, and strong waves crash on it with frequent and relentless regularity. As each wave crashes on the steep shore the water slides down that steep shore with great speed and force, and may actually slip under the next oncoming big wave.

If you are caught in that sliding water, you may or may not go under, but it will sweep you towards the next coming wave, which will take you and slam you back onto that steep shore and you will be swept away again to repeat the cycle. I'll leave it to you to imagine how this can exhaust and drown you. To escape, you have to stay calm and get on your knees or flat on your belly and crawl up that sand out of the power of that water sliding down that steep shore from those big on coming waves. It's a fight. Best to avoid strong curling waves on steep shores: i.e., undertows.

By anon187478 — On Jun 17, 2011

i was almost in a riptide. I'm ten and when we were in Rhode Island, there was a riptide warning and my dad says who cares so we went in and i was being sucked in by a riptide and just when it was about to suck me in and go over me my dad grabbed me. i was not going in for a while.

By anon108419 — On Sep 02, 2010

This article explains well what a riptide and undertow are. However, it downplays an undertow. Undertows are very dangerous. Just last weekend someone off the Texas coast (Matagorda beach) was drowned by an undertow. And it's quite common on Texas beaches.

By Snoopy123 — On Jul 09, 2010

Anon94130- We are vacationing at Huntington Beach, CA soon and I was wondering if you know of any breaks or anything we should worry about. We have only waded on the shoreline, but this trip my husband wants to try boogie boarding.

I am terrified of going through what you did and I couldn’t imagine losing anyone because we didn’t learn how to swim out of tough waters. Thank you.

By anon94130 — On Jul 07, 2010

I was swimming Newport Beach, CA, which has a shore break, and that day was quite violent. I didn't analyze the situation well and went out. I quickly learned the situation was too much for me. The undertow was strong and brought me out just enough for the next wave crash on me. I managed to get out, but I was exhausted. I didn't go out again that day!

By Snoopy123 — On Jun 24, 2010

My family and I vacationed to San Diego a few years ago and spent the day at the beach. We met a family who was visiting California for the first time. The dad was so excited he ran out to the water right away; his wife laughed and said she was glad to see him having fun.

We forgot sunscreen and walked to a surf shop to buy some. On the way back, we saw EMT’s running a patient to the ambulance. It was so sad to see the man on the gurney was that father we met earlier. The mom and kids were crying as they pronounced him dead. Apparently he got taken under by a current. I still remember the sound of them sucking water out of his lungs and seeing his eyes discolored. It had only been about 10-15 minutes.

By anon83436 — On May 11, 2010

Simple and straight. explains well. thanks

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a...

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