When a giant wall of water turns a tranquil river into a raging monster, hold on tight for a wild ride atop a strange hydrological phenomenon known as a tidal bore.
Burton Matthews checks his watch, then scans the placid, empty bay with the dubious eye of a man who makes his living in troubled waters. All around our motorized Zodiac boat, brick-colored mudflats and sandbars glisten in the sun. It’s 2:30 on a mid-July afternoon—just about time. We swing out of the inflatable craft into the shallows of a slack, sediment-filled river now running at barely a trickle. At absolute low tide, the half-mile-wide mouth of the Shubenacadie River is almost completely drained.
“Today’s tide is going to be 26 and a bit,’’ says Matthews. “That means there’s going to be real fast water out there. It’s going to rise really quickly.’’ More precisely, he is talking about a rise of 26-plus feet in just two hours. Hard to believe, but the unique physical characteristics of the Bay of Fundy, of which this river—the longest in Nova Scotia—is a tributary, and the current lunar phase—it’s the fourth day after a full moon—will collaborate in a few minutes to create a rare hydrodynamic phenomenon called a tidal bore. In reality, it’s a genuine tidal wave and the leading edge of a powerful incoming tide forging its way upstream.
Matthews, a third-generation commercial fisherman, notes the ripples in the water where the Shubenacadie drops off into Cobequid Bay. Soon a thin line of foam spreads shoreward from the same spot. In a matter of moments a cresting wave advances up the estuary, moving toward us at running speed. The tide has turned. “It’s one of those things where you can’t hang around too long,’’ says Matthews. “Get back in the boat.’’
With a gathering roar, the bore scatters shorebirds as it sweeps across the sandbars. It resembles a wave crashing on a beach yet somehow surges forward without dispersing. We cinch our life jackets as the wave, about three feet on the face, tosses our Zodiac and keeps thundering upriver. Matthews revs up his 60-horsepower outboard engine and powers ahead through the now-churning river. In the space of just two hours, millions of gallons of seawater will be pushed nearly 30 miles inland, converting the calm Shubenacadie into a raging monster filled with class IV rapids. As I grab a line to keep from falling overboard, I marvel at the sudden transformation I’ve just witnessed. How on earth does such a thing happen?
Derived from the Old Norse word bara, or “wave,’’ a tidal bore is an elusive event. Hubert Chanson, an engineering professor at the University of Queensland, in Brisbane, Australia, is one of the few people to study the phenomenon. He estimates that roughly 200 rivers worldwide have bores. Chanson lists several specific conditions for a bore to occur: The tidal range must be larger than 13 to 19 feet, and the river must be shallow, with a gradually sloping bottom and a broad, funnel-shaped estuary. But bores are mysterious creatures. Depending on the lunar phase and the topography of a river’s mouth, Chanson explains, they may form a handful—or hundreds—of times annually. Bores are most powerful around a new or full moon, when tidal amplitude is greatest; every 206 days the perigee of the moon’s oval-shaped orbit coincides with these lunar phases, creating extra-large spring tides and, depending on local conditions, particularly impressive bores. Conversely, they are rarely seen at neap tides, when the moon is at its apogee. In isolated instances, storms and tsunamis can also induce bores.
With the exception of Antarctica, every continent has a smattering of such rivers, though the phenomenon goes by many names: le mascaret, on France’s Gironde River, the benak, on Sarawak’s Lupar River, and the pororoca, on numerous Amazon tributaries in Brazil. In the United States perhaps the best-known bore occurs in the Turnagain Arm of Cook Inlet, in Alaska.
“They’re not well understood,’’ says Don Thiederman, a Hull, Massachusetts, carpenter who also runs the Tidal Bore Research Society, a clearinghouse devoted to the wave. “In my opinion it’s the most underrated of any type of event that Mother Nature has produced. You have people looking for volcanoes and geysers, climbing tall mountains. But to me, tidal bores are just plain fascinating.’’
In North America one of the best places to see bores is eastern Canada’s Bay of Fundy, between New Brunswick and Nova Scotia. The bay, famed for having the world’s greatest recorded tides, is fed by several ideally shaped rivers, including the Shubenacadie, the Salmon, and the Petitcodiac. Fundy’s bathymetry is crucial: At its mouth, the bay is approximately 60 miles wide and more than 400 feet deep. It then runs 180 miles eastward, gradually tapering and shallowing, before finally splaying at its head into two narrowing arms—Chignecto Bay to the north and Minas Basin to the south—that leave the incoming seawater nowhere to go but up.
Fundy’s length is key, says Richard Faulkner, a retired outdoor educator in New Brunswick. Every body of water has a natural frequency of oscillation; once set in motion, its waves move back and forth in a regular, fundamental period. In the case of the Bay of Fundy and the adjoining Gulf of Maine, the fundamental period is a little more than 13 hours, which dovetails nicely with the tidal interval of 12 hours, 25 minutes. This phenomenon is called seiche—a fancy French word for “bathtub effect.’’
“If you sit in a bathtub and wiggle your hips and rear end, a wall of water goes to the front of the tub,’’ says Faulkner, who operates a Fundy sea-kayak company. “If you wiggle again as the wave comes back, it’s amplified.’’
At Fundy, the seiche induces a huge tidal range and amplifies the energy to help propagate tidal bores. Roughly 15 miles west of the Shubenacadie, at the coastal farming community of Burncoat, the highest tides in the world—an astounding 53.38 feet—have been recorded.
In the outer bay, the tidal action helps to stir deep, nutrient-rich water toward the surface, attracting four species of baleen whale—including the highly endangered northern right whale—that feed on krill in summer and fall. In the upper bay, the flushing exposes 620 square miles of ocean floor at low tide. In late summer these enormous mudflats are crawling with tiny invertebrates: an irresistible takeout meal for millions of migrating shorebirds, particularly plovers and the majority of the world’s population of semipalmated sandpipers. Fundy is the lone pit stop these birds will make on a long southern migration from their Arctic breeding grounds to wintering areas in Central and South America.
The seiche helps propel deep-water swells into the shallows, where compression stacks the waves higher. When a wave’s height exceeds half the water depth, the crest overtakes the trough and breaks into a frothing wall of water, forming a bore. Under the right circumstances, these tidal waves can even become killers. In 1922 a powerful 15-foot bore on the Colorado River, known locally as the burro, capsized a passenger ship bound for Yuma, Arizona, killing 86 people. Somerset Maugham very nearly suffered the same fate on Sarawak’s Lupar River a year earlier; the terrifying incident inspired one of his finest short stories, “The Yellow Streak.” Over the centuries the so-called Silver Dragon bore on China’s Qiantang River—at 30 feet, the world’s tallest—is thought to have swept thousands of spectators and daredevil swimmers to their deaths.
If there’s a big wave, however, intrepid surfers are bound and determined to catch it. These “bore riders’’ flock annually to En-gland’s Severn and France’s Dordogne rivers. Every spring São Domingos do Capim, a small Brazilian town on a tributary of the Amazon and nearly 200 miles from the Atlantic Ocean, holds a pororoca surfing competition. Longboarders brave the wave, along with the Rio Capim’s piranha and caiman, and hang ten for miles through the jungle on rides that can exceed half an hour.
From late spring through the early fall, three outfitters offer tidal-bore whitewater rafting trips on Nova Scotia’s Shubenacadie River. The thrill is like a roller-coaster as we hurtle along in the Zodiac past the bulwarks of centuries-old Acadian dykes, bluffs of sandstone, and nesting pairs of bald eagles. A bit disconcerting, too: Incredibly, we’re racing upstream at more than 15 miles per hour. Swells slap the banks and ricochet into the middle of the channel, where they collide with enormous standing waves created by the massive influx of seawater flowing over the sandbars. Matthews studies the turbulent river, then digs in the engine. “This is where it’s going to be fun,’’ he says. “Just hold on tight.’’
We move out into the channel and are instantly swept through an enormous wave train stretching more than 600 feet. Water crashes down on us from all angles of the pounding, bucking river—and then we meet a monster wave. The nose of our 16-foot craft rises scarily close to vertical, and then finally clears the crest, only to send us diving into a maw of foam that fills the Zodiac to the gunwales before finally spitting us upriver. Somehow we’re all still in the boat. “Those were big,’’ Matthews says. High praise from a waterman.
But the wild, class IV rapids quickly flatten out as the tide deepens the river by the minute, minimizing the hydraulics. So we speed to the next stretch of sandbars, where the rising tide creates another temporary set of washing machine–like waves. Just beyond a cantilevered highway bridge, where the bluffs constrict the Shubenacadie into a narrow channel, Matthews points out an ideal spot to catch a large bore. “I came through here two years ago,’’ he recalls. “The first boat that hit it surged into a 12-foot wall of water. The bore flipped the boat over backwards.’’
Fundy’s tidal turmoil has also carved New Brunswick’s fantastically shaped Hopewell Rocks and ignited the renewable-energy dreams of power companies. Opened in 1984, a tidal-generating station at Annapolis Royal, Nova Scotia, the only one of its kind in North America, generates 30 million kilowatt-hours per year—enough electricity to supply 4,500 homes—by channeling water through a 25-foot-diameter turbine during the ebb tide. By next summer underwater hydrokinetic tidal turbines may be tested in the narrow Minas Channel, which connects the Bay of Fundy to Minas Basin. The demonstration project could potentially generate 333 megawatts of electricity—enough to power 45,000 homes. An earlier scheme, a five-mile dam outfitted with turbines that would have completely impounded Cobequid Bay and decimated its bores, was successfully halted in the early 1980s over environmental concerns that the barrage would wipe out the Atlantic shad fishery and drastically shrink the intertidal zone—a globally important area for migrating shorebirds.
Elsewhere, however, human development has destroyed several famous bores. Repeated dredging of France’s Seine River has eliminated the hazardous le mascaret, which once approached 24 feet. Dams and irrigation projects have virtually eradicated the Colorado’s once-lethal burro. Another prominent bore casualty lies on Fundy’s north shore, where a tidal wave that could top six feet used to race 25 miles up New Brunswick’s Petitcodiac River. But in 1968 a road causeway was built just outside the city of Moncton, effectively damming the river.
The ensuing changes to the Petitcodiac were nothing short of a disaster. “When you put up a barrier it’s just about the most stupid thing you can do to a tidal river,’’ says Daniel LeBlanc, recently retired from the Moncton-based environmental group, Petitcodiac Riverkeeper. “There’s no flushing anymore. Each tide brought a gigantic load of silt, and that’s accumulated for close to 40 years. Over 90 percent of the river is filled below the causeway. The silt now extends 35 kilometers [about 22 miles] downstream and continues to be deposited with every tide. The estimate is 2.6 million cubic yards of new silt a year; it’s a phenomenal amount.’’
Since the silting, the Petitcodiac’s tidal wave, once Moncton’s top natural tourist attraction, has become a ripple that rarely rises a foot and a half. “It was an extraordinary destruction of a river system,’’ LeBlanc says. “Not just visually but in the life of the river.’’
Ten native fish species, including gaspereau (also known as alewives) and Atlantic sturgeon, were either eliminated or significantly reduced, LeBlanc says. Major shad and Atlantic salmon runs vanished within a decade, and dozens of fishermen lost their livelihoods. The silt accumulation now extends into Shepody Bay, choking the mudflats and their mud shrimp, prompting shorebirds to search elsewhere for food. Subtle changes have even come to the iconic Hopewell Rocks, where large areas of the rocky seafloor are now covered with silt at low tide.
Help could be on the horizon. In 2005 a Canadian government study concluded the structure needed to be dismantled to restore the Petitcodiac’s full tidal flow. A final decision on the rehabilitation project, which will cost an estimated $33 million to $47 million, has yet to be made, but LeBlanc is optimistic that the river—and its bore—will once again run free. “The unknown is how large the bore will be,’’ he says. “Those predictions vary from exactly the same height as before, to just little bit under or more. There’s a sense it’s going to be very good—and hopefully as good as it was before the causeway.’’
While the Petitcodiac’s advocates dream of restoring the river to its natural state—and perhaps even attracting bore-riding surfers —the Schubenacadie flows on. Matthews pilots the Zodiac to the Tidal Bore Rafting Park’s floating dock and disembarks his waterlogged passengers. The muddy river banks that sucked at our shoes when we boarded the boat four hours ago now lie beneath 26 feet of nearly calm seawater. It won’t be long before the Shubenacadie begins to ebb. Come three o’clock in the morning, beneath a moon-filled night, the sea will again rise up singing. Just like clockwork.
Travel writer Christopher R. Cox is the author of Chasing the Dragon: Into the Heart of the Golden Triangle.
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The Nature of Tidal Bores
The Bay of Fundy, which holds the world record for the greatest difference in water levels between low and high tides, is one of the most thrilling places in the world to experience a tidal bore. Like every body of water, Fundy has a natural frequency of oscillation; once set in motion, its waves move back and forth in a regular, fundamental period. This phenomenon is called seiche—French for “bathtub effect.” Scooting forward in a bathtub makes the water slosh forward as a single wave. If you push forward again as the wave goes by, it becomes larger. Such an amplifying effect is greater in a shallow river, such as Nova Scotia’s Shubenacadie, with a gradually sloping bottom and a broad, funnel-shaped estuary. The incoming ocean tide enters the basin moving toward the mouth of the river. If the time between high tides is about the same as the period it takes the wave to go from one side of the basin to the other, this natural synchronization bolsters the wave’s size. The seiche propels deep-water swells into the river’s shallows, where the compression stacks the waves higher. When the wave’s elevation exceeds half the water’s depth, the crest overtakes the trough and breaks into a tumultuous wall of water, forming a tidal bore that continues to move upstream. These hydrological marvels tend to be most powerful around a new or full moon, when tidal amplitude is at its greatest.—C.C.