Situated on the Bay of Fundy, the Reversing Falls of Saint John are subject to the immense changes in the remarkably powerful local tides which actually cause the waters to change course multiple times throughout the day.
The Bay of Fundy is home to some of the highest tides in the world, and can have variations from high to low tide upwards of 16 meters, roughly the same height as a five story building. Due to these drastic surges, when the powerful tides come up, the river and waterfalls change direction in the region of the Reversing Falls thanks to the unique underwater geography. The tides of the bay are semidiurnal, meaning the tide rises and falls about once every 12 hours, and if visitors can afford to see the waters at both high and low tide they will be best be able to observe the shift.
Due to the turbulence of the Reversing Falls, boats steer well clear of the roiling epicenter, but tour boats have been known to bring visitors close to the maelstrom, although it seems that most of these tours are no longer in operation.