A battle of water
At Saint John, New Brunswick, the 673-kilometer-long Saint John River ends at the Bay of Fundy. Here, it is forced through a narrow, rocky gorge, the site of a twice-daily hydrological battle. The Bay of Fundy experiences the highest tides on Earth, with tidal ranges that can exceed 16 meters (53 feet). In the Saint John Harbour, the tide reaches up to 8.5 meters (28 feet). This immense rise and fall of seawater, a cycle that takes about 12.5 hours, determines the river's behavior.
During low tide, the river's water level is higher than the bay's, and it flows downstream into the ocean, creating a turbulent series of rapids and whirlpools over underwater ledges. As the tide comes in, the powerful surge of the bay's water slows the river's exit, eventually stopping it entirely. This brief period of equilibrium, known as slack tide, lasts for only about 20 minutes and is the only time vessels can safely pass through the gorge. After this calm, the bay's water level surpasses the river's, forcing the Saint John River to flow upstream. This reversal creates a new set of rapids, this time flowing inland, which peak at high tide when the bay's water is about 4.4 meters (14.5 feet) above the river.
A story told in stone
The dramatic landscape of the Reversing Falls comes from immense geological forces and ancient history. The gorge itself is a fault line, at the collision point of two ancient continental fragments. The rocks on one side are billion-year-old Precambrian marble, originally part of a proto-South America, while the other side consists of much younger, 500-million-year-old Cambrian sandstone and shale from what was once part of North Africa. These fragments were pushed together hundreds of millions of years ago during the formation of the supercontinent Pangea.
The present-day channel was carved more recently. Before the last Ice Age, the Saint John River met the sea through a different route. Glacial moraines dammed that original outlet, forcing the river into its current path through the hard rock gorge about 15,000 years ago. Initially, this created a massive three-tiered waterfall. As glaciers receded and sea levels rose, the waterfall was drowned, but its remnants exist as a dramatic underwater topography. The riverbed drops 25 meters into a plunge pool just past the islands in the gorge, and then falls again by another 40 meters, creating the deep, turbulent conditions that form the rapids. The modern reversing phenomenon is only about 3,000 years old.