How Do Tidal Bores Work?

Tidal bores are huge, fast waves that travel the wrong way up rivers, and they only form during very specific circumstances. Learn how they work in this episode of BrainStuff. 

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Episode Transcript

Speaker 1 (00:01):
Welcome to brain Stuff production of I Heart Radio, Hey
brain Stuff, Lauren vocal bomb. Here. The Alaskan coast is
as dramatic as it is vast, and it spans over
thirty three thousand miles or over fifty four thoud kilometers.
U s State owes its very name to this sweeping ocean. Border.
Alaska is an English language corruption of an Alouette term that,

(00:25):
according to one translation, means the shore where the sea
breaks its back. And if you drive out to a
place called turning in Arm in the Gulf of Alaska
at just the right time, you get to watch some
aquatic action that's pretty spectacular. The turnig In Arm Waterway
is a northern branch of the cook Inlet that cuts
into the Greater Anchorage area. Here, the water normally flows

(00:45):
out toward the inlet, but shortly after low tide waves
move in the opposite direction and travel upstream, and these
waves can be epic in scale. The biggest waves are
up to ten ft or three meters tall. This is
a well known example Boar tides also known as tidal
bores or just boars, and riding them has become a
rite of passage for many surfers in the Pacific Northwest.

(01:09):
While this phenomenon has been seen at numerous locales around
the world, it requires a very specific set of conditions.
But before we dive into those, it might be a
good idea to go over some basic tidal science. By
and large, tides are gravity's handiwork. Due to the gravitational
influence of our moon plus the inertia of our movement
through space, the ocean is always bulging out a bit

(01:31):
on opposite sides of the planet. While Earth rotates, it
passes through these tidal bulges. That means your favorite seaside
beach will experience high tide when it enters each bulge,
and low tide as it travels between them, So plan
your sandcastle contests accordingly. Because Earth finishes a new rotation
around its axis once every twenty four hours, most coastal

(01:52):
areas witness two high tides and two low tides per day,
but there are loads of exceptions. The layout of continents, islands,
and peninsulas impedes tides in certain places. Parts of the
Gulf Coast, for example, only see one daily set of
high and low tides. No matter where you are, though,
the tides will vary in strength from day to day

(02:14):
all over the world. During full moons and new moons,
when the Earth, moon and Sun are arranged in a
straight line, high tides are especially high and low tides
are really low. We call these extreme tides spring tides, which,
despite the name, occur all year round, not just in
the springtime. But let's turn back to turn Agin Arm.
Although it's shallow and narrow, the Turnigin feeds into a

(02:36):
broad bay, namely cook Inlet. On this wide expanse, the
difference between high and low tides, i e. The tidal
range can be stark. The cook inlets water level at
high tide maybe thirty five ft or ten and a
half meters higher than it was during the preceding low tide,
and these are exactly the ingredients needed to produce tidal bores.

(02:57):
After a low tide on cook Inlet, water funnels into
the herning In Arm, generating boar waves that barrel inland,
sometimes traveling at speeds up to twenty four miles per hour.
That's nearly thirty nine kilometers per hour. Since bores only
happen when the circumstances are just right, not all waterways
that touch the Ocean can get them. In total, there
are eighty or so rivers around the globe that undergo

(03:18):
tidal bores. The phenomenon tends to be at its most
dramatic during spring tides, whereas the turning in Arm has
twice daily bore tides. The Amazon River only receives bores
on the days of new and full moons. Once the
spring tides arrive, water from the Atlantic comes charging up
the Amazon River, temporarily reversing its natural flow. Waves generated

(03:40):
by this process have been known to journey about five
hundred miles or eight hundred kilometers inland, called porta rocca.
The Amazon bars attain considerable size, measuring upwards of thirteen
feet or four meters tall in some cases. The strongest
portor roca events of all take place every year on
the equinoxes. To prepare for these surges, local residents move

(04:00):
their boats and livestock safely away from the Amazon. Accidents
do still happen, though Jacques Cousteau once lost a boat
to pororoca waves while filming in South America. Title boars
impact ecosystems wherever they occur. Trees, rocks, and river bottom
sediment get pushed around by the waves, and those aren't
the only things they churn up. Down in the Amazon,
current reversals can leave stunned or dead animals floating in

(04:23):
the water, attracting hungry piranhas. Strong boars are also prone
to eject fish from rivers, stranding them on beaches or
launching them into the air. That's why Alaskan bald eagles,
Australian sharks, and Malaysian crocodiles like to scavenge in the
wakes of title boars. Big boars attract humans as well.
Sal Domingo Stolkapim, a community that borders the Amazon, has

(04:43):
been hosting the Brazilian National Poroca Surfing Championship since n
Alaskan wave chasers flocked the turning in arm shortly before
or after every spring tide when the boars are at
their largest. Oh and there's one last thing we should
mention about boar tides. They roar. The waves produce a
great deal of turbulence, wildsplacing sediment, scraping up shoals, and

(05:05):
unleashing loads of air bubbles. Add all those sources up
and you've got a recipe for thunderous low frequency booms
that can be heard over vast distances. Today's episode was
written by Mark Bancini and produced by Tyler. Playing brain
Stuff is a production of I Heart Radio's How Stuff Works.
For more on this and lots of other swell topics

(05:26):
you know, visit our home planet, how stuff Works dot com.
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