The phenomenon of the Reversing Falls is caused by the tremendous rise and fall of the tides of the Bay of Fundy, which are the highest in the world. The natural southward-facing opening of the Bay of Fundy receives the on rushing ocean tides directly like a funnel. The tidewater is normal when it enters the bay at its widest point. But the farther up the bay it travels the more it changes. It is, in effect, squeezed by the ever-narrowing sides and the constant shallowing of the bottom forcing the water higher up the shores. The low tide running out of the bay collides with the new, incoming high tide, combining forces to make a higher wave coming in. The combination of wave forces is called resonance.
The Bay of Fundy tides are a result of the tidal action originating in the Southern Indian Ocean sweeping around the Cape of Good Hope and then northward into the Bay of Fundy. They are also affected by the distance of the moon from the earth at this longitude. Here in Saint John, the bay tides rise 28 ½ feet. When the tide is low, the St. John River, 450 miles long, empties into the bay. Between Fallsview Park and the Pulp Mill, the full flow of the river thunders through a narrow gorge. An underwater ledge, 36 feet below the surface causes the water in the river to tumble downward into a 175-200 foot deep pool. Below the mill and under the bridge. The water then boils in a series of rapids and whirlpools.
As the bay tides begin to rise, they slow the course of the river and finally stop the river's flow completely. This short period of complete calm is called slack tide. It is only at this time that boats are able to navigate the Falls. Shortly after this slack tide the bay tides become higher than the river level and slowly, at first, the river begins to flow upstream. As the bay tides continue to rise, the reverse flow gradually increases and the rapids begin to form, reaching their peak at high tide. The effect of this reversal is felt upstream as far as Fredericton, more than 80 miles inland. At this point the tidal waters are actually 14 ½ feet higher than the river.
After high tide the bay tides begin to fall and the upstream flow of the river gradually lowers until the bay tides fall to the level of the river - once again resulting in another slack tide. The river then resumes its normal course and begins to flow back out of the bay. The bay tides continue to fall below the level of the river until at low tide the rapids are again at their peak, flowing down stream. At this point the tidal waters are 14 ½ feet lower than the river level.
The tide rises and falls once about every 12 and a half hours. To best appreciate the Reversing Falls, you should try to view them at least twice on a given day - near low tide and near high tide. A good water level vantage point from which to view the Falls is Fallsview Park, not far from the Reversing Falls Information Centre. A twelve-minute film presentation explaining the phenomenon of the Reversing Falls, and highlighting various attractions in the Saint John area, can be viewed in the Reversing Falls Roof Top Theatre on the observation deck. A small admission charge applies to view the film. [SOURCE]
pretty much the WORST tourist attraction in the world, because it would be hard for someone that doesnt live here, to predict the tides, to be able to view both high and low tides.
(Reversing Falls, Low Tide | NOTE* the disgusting grey sky)