The mechanics of a reversing river
Twice every 24 hours and 50 minutes, the Petitcodiac River in New Brunswick stops flowing towards the Atlantic and begins to flow backward. This reversal is driven by the immense power of the Bay of Fundy tides, which are the highest in the world, with a vertical range that can exceed 16 meters (52 feet). The phenomenon begins with 160 billion tonnes of seawater surging into the bay, a volume greater than the combined flow of all the world's freshwater rivers over the same period.
The unique funnel shape of the Bay of Fundy is a primary cause of these extreme tides. As the tide from the Atlantic Ocean enters the bay's wide mouth, it is forced into an narrow and shallow space, causing the water level to rise dramatically. This effect is amplified by a principle known as tidal resonance. The bay's natural period of oscillation—the time it takes for water to slosh back and forth, is between 12 and 13 hours. This timing is nearly in sync with the 12-hour and 25-minute cycle of the lunar tide, creating a reinforcing effect much like pushing a swing at the perfect moment to make it go higher.
This massive influx of water is forced into the rivers that drain into the bay. In the Petitcodiac, the leading edge of the incoming tide forms a distinct wave, known as a tidal bore. This wave travels upstream at speeds of 5 to 13 kilometers per hour, overpowering the river's natural downstream current and reversing its flow. The bore itself can range from a few centimeters to over a meter in height, and within an hour of its arrival, the water level can rise by 7.5 meters.
A river restored
For decades, the Petitcodiac's tidal bore was nearly nonexistent. In 1968, a rock-and-earth causeway was constructed between Moncton and Riverview, severely restricting the river's flow. The structure acted as a dam, causing an estimated 10 million cubic meters of silt to accumulate downstream within just three years. The once-powerful bore, which had historically reached heights of up to two meters, was reduced to a mere ripple.
After years of advocacy, the causeway gates were permanently opened in April 2010, marking the beginning of a significant restoration project. The results were almost immediate. The renewed force of the tides began to scour the accumulated silt, widening and deepening the river channel. Within months, the channel had doubled in width in some areas, and fish species that had been absent for decades began to return.
The tidal bore was reborn. Its size and power have been steadily increasing since the causeway's removal and its replacement with a 240-meter steel girder bridge in 2021. This engineering project has restored a unique hydrological phenomenon and revived a local culture around it. Surfers now frequent the river, and in 2013, a team set a record by riding the bore for 29 kilometers.