I can’t decide who is cuter- Boo from Monsters, Inc. or baby Dory from Finding Dory.  I mean, just look at this face:



Anyways, like her older self, Baby Dory suffers from short-term memory loss.  Because she has difficulty forming new memories, her parents try to teach her through practicing situations and repeating songs and rhymes.  One of the lessons they try to teach Baby Dory is to avoid the undertow: “We see the undertow and we say heck no!”  Unfortunately, this one doesn’t seem to stick- instead of saying “heck no,” Baby Dory says “let’s go!”  Later, poor Baby Dory gets dangerously close to the undertow and is swept out to sea, away from her home at the Marine Life Institute.

So what is an undertow?

An undertow is a current below the surface that moves away from the shore.  When a wave breaks on the shore, water is pushed up to the shore.  Water from previous waves is pushed down, underneath waves coming in.  This creates a current running back out to sea as the water tries to level out.


Undertows can chip away at shorelines.  Turbulent storm waters create strong undertows, which can pull sand off beaches, eroding the shoreline.  By studying and modeling currents, researchers aim to predict how undertows shape the shore.  With this information they can make suggestions to prevent erosion through building sand dunes or other methods.

Now, I’ve been talking about ocean currents, but Baby Dory is from the Marine Life Institute, not the ocean.  Despite this, it’s still possible to have currents in aquariums.  The Marine Life Institute is based on the real-life Monterey Bay Aquarium.  The aquarium uses pumps and surge machines to generate water movement in their exhibits.  Sometimes these “artificial” currents are referred to as undertows, which is where the little rhyme in Finding Dory likely came from.

Undertows are usually considered harmless.  Most of the time, swimmers are able to simply stand in an undertow and feel sand shifting under their feet.  Undertows typically do not pull swimmers offshore into deep water, but they may still be dangerous for small children or anyone who can’t walk up against the flow.

In other words, they can be very dangerous to tiny, adorable fish who suffer from short-term memory loss.


Though the names are often used interchangeably, rip currents are not undertows.  They start out similar to undertows- waves break on the shore and water is pushed back out to sea.  The difference is that rip currents are the result of a non-uniform flow.  For example, if there is a break in a sandbar, water can flow back through the gap much more easily than over the sandbar.  As the water flows through this path of least resistance, it creates a strong current.  These strong currents are much more dangerous than undertows.  According to the National Oceanic and Atmospheric Administration, in 2015 half of fatalities that occurred in surf zones in the United States were caused by rip currents.


If you’re ever caught in a rip current, you should go with the flow.  Swimming directly towards the shore through the opposing rip current can be energy draining.  If the current is weaker, you may be able to escape the current by swimming parallel to the shoreline.  If the current is stronger, you should float as the current carries you farther out.  Eventually the rip current will die down and you can swim parallel to the shore some distance and then pack to shore.  Of course, you should never swim alone, and if there’s a lifeguard present you should gain their attention for help.

Sorry Dory, but sometimes the best thing to do is not to “just keep swimming,” but to just float.


Keep calm and science on.

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