The Reversing Rapids is a unique phenomenon caused by the tremendous tide in the Bay of Fundy; the highest tides in the world. The bay's tidal action is affected by the funnel shape of the bay and by the moon's phases.
At low tide, the St. John River, which runs for 725 km (450 mi.) through New Brunswick, empties into the bay through a narrow rocky gorge. Near Fallsview Park, an underwater ledge 11 m (36 ft.) below causes a boiling series of rapids and whirlpools. The rising tide slows the river current to a stop and for 20 minutes a rest period called low slack tide allows boats to navigate the rapids. Once the tide is higher than the river, the reversal of the current occurs and continues until high tide. The water rises up to 4.5 m (14.5 ft.) above sea level in the gorge.
Then the tide slowly descends but still flows inland until it is at river level again (high slack). After 20 minutes rest the river resumes its natural flow and the tide drops away to low tide, revealing the rapids and whirlpools again.
It is recommended to view the rapids twice; near low tide and near high tide. View this wonder from 2 vantage points - Fallsview Park and the Falls Restaurant observation deck.