What happens here ?
Reversing Falls Rapids is the world’s greatest example of tidal impact upon a river !
Twice daily, 100-billion tons of water … a volume equal to all of the world’s rivers … enters and exits the bay. At this location water levels change as much as eight-and-a-half meters … or 28 feet in a tidal cycle. Reversing Falls Rapids experiences two tidal cycles daily resulting in 2 low tides and 2 high tides. Each complete cycle is roughly 12 hours. In-between the tidal extremes is a calm period known as “slack tide”, this condition lasts for just 20 minutes. This is the only time vessels can safely pass through the gorge.
You may wonder why this is called Reversing FALLS Rapids. If you could see beneath the water, you would find rocky ledges near the surface stretching from the pulp mill to the Reversing Falls bridge … these create the rapids.
Then immediately after the bridge the river bed plunges 61 meters into a massive pool … deep enough to cover the 15-storey Saint John City Hall building.
Interestingly, five thousand years ago, sea level was 100 feet lower. Native people living here at that time enjoyed an impressive waterfall !
Reversing Falls Rapids is a tidal wonder. But this area has an equally incredible and much older geological story. Two ancient continents meet here in the gorge. The dark volcanic rock originated in today’s Africa and is about 500-million years old and the light grey rock is marble formed from ancient see creatures. It is about 1.2 billion years old and originated near the South Pole !
The gorge through which the Reversing Falls Rapids flows was created 11,000 years ago. As the ice ages receded and water flows increased off the land the Saint John River found a new path to the ocean.
Today the Saint John River has a watershed of 56-thousand-kilometres watershed all of which passes through the gorge here at the Reversing Falls Rapids !