Spanning 33,904 miles (54,563 kilometers), the Alaskan coast is as dramatic as it is vast. The 49th U.S. state owes its very name to this sweeping ocean border. "Alaska" is an English language corruption of an Aleut term that — according to one translation — means "the shore where the sea breaks its back." If you drive out to a place called Turnagain Arm in the Gulf of Alaska at just the right time, you'll get to watch some aquatic action that's pretty spectacular.
A northern branch of the Cook Inlet, the Turnagain Arm waterway cuts into the greater Anchorage area. Here the water normally flows out into the inlet, but shortly after low tide, waves move in the opposite direction and travel upstream. These can be epic in scale; the biggest waves are 6 to 10 feet (1.8 to 3 meters) tall. Riding them has become a rite of passage for many surfers in the Pacific Northwest.
Turnagain's back-tracking waves are a well-known example of bore tides, also known as "tidal bores" or just "bores." 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, the ocean is always bulging out a bit 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 the bulges and low tide as it travels between them. So plan your sand castle contests accordingly.
Because Earth finishes a new rotation around its axis once every 24 hours, most coastal 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. 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 — they don't just happen in springtime).
Going Against the Flow
Now let's head back to Turnagain Arm. Although it's shallow and narrow, the Turnagain feeds into a broad bay: namely, Cook Inlet. On this wide expanse, the difference between high and low tides — i.e.: the "tidal range" — can be stark. Indeed, the Cook Inlet's water level at high tide may be 35 feet (10.6 meters) higher than it was during the preceding low tide.
Guess what? We've just checked off all the ingredients needed to produce tidal bores. After a low tide on Cook Inlet, water funnels into the narrow Turnagain arm, generating bore waves that barrel inland, sometimes traveling at speeds of up to 24 miles per hour (38.6 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 80 or so rivers around the globe that undergo tidal bores. The phenomenon tends to be at its most dramatic during spring tides, which actually monopolize the bore tides of some rivers.
Whereas the Turnagain Arm and Malaysia's Batang Lupar River have twice-daily bore tides, the Amazon River only receives bores on the day of a new or full moon. Once the spring tides arrive, water from the Atlantic comes charging up the Amazon River, temporarily reversing its natural flow. Waves generated by this process have been known to journey 497 miles (800 kilometers) inland. Called "Pororoca," the Amazon bores attain considerable size, measuring upwards of 13.1 feet (4 meters) tall in some cases. The strongest Pororoca events of all take place every year on the equinoxes.
To prepare for these surges, local residents move their boats, livestock and other valuables safely away from the Amazon. Accidents still happen, though: Jacques Cousteau once lost a boat to Pororoca waves while filming in South America.
Surf's Up at Suppertime
Other notable bore-prone waterways include Australia's Styx River and the Qiantang River in China. Tidal bores 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 leave stunned or dead animals floating in the water, attracting hungry piranhas. Strong bores 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 tidal bores.
Big bores attract human surfers as well. São Domingos do Capim, a community that borders the Amazon, has been hosting the Brazilian National Pororoca Surfing Championship since 1999. Alaskan wave-chasers flock to the Turnagain Arm shortly before or after every spring tide, when the bores are at their largest. If you'll pardon the pun, that sounds like a swell time.
Oh and there's one last thing we should mention about bore tides: They roar. The waves produce a great deal of turbulence, while displacing sediment, scraping up shoals and 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.