Buzz
In July 2014, surfers caught the bore tide at Alaska's Turnagain Arm. This location, part of the Cook Inlet's lower arm, is famous for its bore tides, where waves can surge to heights of 6 to 10 feet (1.8 to 3 meters) and travel at speeds of 10 to 15 mph (16 to 24 kph). Streeter Lecka/Getty Images captured this thrilling event.The Alaskan coastline stretches an impressive 33,904 miles (54,563 kilometers), offering a dramatic and expansive seascape. The state's name, derived from an Aleut term, translates to "the shore where the sea breaks its back," reflecting its rugged maritime heritage. Visitors to Turnagain Arm in the Gulf of Alaska can witness breathtaking aquatic displays during specific tidal conditions.
Turnagain Arm, a northern extension of the Cook Inlet, lies near Anchorage. Typically, water flows outward into the inlet, but after low tide, waves reverse direction and surge upstream. These waves can reach remarkable heights of 6 to 10 feet (1.8 to 3 meters), making them a popular challenge for surfers in the Pacific Northwest.
The reversing waves at Turnagain Arm are a famous instance of bore tides, often referred to as "tidal bores" or simply "bores." This natural event occurs in various locations globally but depends on a very specific combination of factors. Before exploring these conditions, it’s helpful to review some fundamental tidal principles.
Tides 101
Tides are primarily the result of gravitational forces. The moon’s gravitational pull causes the ocean to swell outward on opposite sides of the Earth. As Earth rotates, it moves through these "tidal bulges," leading to high tide when a coastal area enters a bulge and low tide as it exits. This cycle is essential to consider when planning seaside activities.
Since Earth completes a full rotation every 24 hours, most coastal regions experience two high tides and two low tides daily. However, exceptions exist due to the influence of landforms like continents, islands, and peninsulas. For instance, some areas along the Gulf Coast only observe one high and one low tide each day.
Regardless of location, tidal intensity fluctuates daily. During full and new moons, when the Earth, moon, and sun align, high tides rise exceptionally high, and low tides drop significantly. These extreme tides, known as "spring tides," occur throughout the year, not just during the spring season.
Going Against the Flow
Returning to Turnagain Arm, this shallow and narrow waterway connects to the expansive Cook Inlet. The tidal range here is significant, with water levels at high tide rising up to 35 feet (10.6 meters) above the previous low tide, creating a dramatic contrast.
These conditions are perfect for tidal bores. After low tide in Cook Inlet, water rushes into the narrow Turnagain Arm, forming powerful bore waves that surge inland at speeds reaching 24 miles per hour (38.6 kilometers per hour).
Tidal bores are rare, requiring precise conditions. Only about 80 rivers worldwide experience this phenomenon. Bore tides are most intense during spring tides, which dominate the tidal patterns of certain rivers.
While Turnagain Arm and Malaysia's Batang Lupar River see bore tides twice daily, the Amazon River experiences them only during new or full moons. During spring tides, Atlantic waters surge up the Amazon, temporarily reversing its flow. These waves, known as "Pororoca," can travel up to 497 miles (800 kilometers) inland, reaching heights of over 13.1 feet (4 meters). The most powerful Pororoca events occur annually during the equinoxes.
To safeguard against these powerful surges, locals relocate their boats, livestock, and valuables away from the Amazon. Despite precautions, accidents occur; for instance, Jacques Cousteau once lost a vessel to Pororoca waves while filming in South America.
Surf's Up at Suppertime
Other rivers prone to tidal bores include Australia's Styx River and China's Qiantang River. These bores significantly affect local ecosystems, displacing trees, rocks, and sediment. In the Amazon, the reversal of currents leaves dead or stunned animals floating in the water, drawing piranhas. Powerful bores can also fling fish onto beaches or into the air, providing opportunities for Alaskan bald eagles, Australian sharks, and Malaysian crocodiles to scavenge.
Human surfers are also drawn to these massive waves. Since 1999, São Domingos do Capim, near the Amazon, has hosted the Brazilian National Pororoca Surfing Championship. In Alaska, surfers gather at Turnagain Arm around spring tides when the bores are at their peak. It’s an exhilarating experience, to say the least.
One final note about bore tides: They roar. The waves create immense turbulence, displacing sediment, scraping shoals, and releasing air bubbles. These factors combine to produce thunderous, low-frequency booms that can be heard from miles away.
On March 10, 2016, an Australian surfer achieved a world record by riding a bore wave for 10.6 miles (17.2 kilometers) along Indonesia's Kampar River. This remarkable feat set the record for the longest distance ever surfed on a river bore.
