Ah, the 'Reversing Falls', one of the more interesting little attractions of Saint John, but please DO NOT compare it to the 'Niagara Falls' . . . they’re very different indeed.
So, what are the 'Reversing Falls'? Well, they are a unique phenomenon caused by the highest tides in the world, those of the 'Bay of Fundy'. Without getting too technical, at low tide, the 'St John River' empties into the bay through a narrow rocky gorge. This steep drop causes a series of rapids and whirlpools where the river meets the bay. Now, as the bay’s tide rises to meet the river, the current of the river stops to a standstill and will actually reverse once the tide is higher than the river. After the bay’s tide slowly descends, the river resumes its natural flow and the tide drops away to reveal the rapids and whirlpools again. It is recommended to view the falls twice -- once near low tide and again near high tide.
Whilst here, it is a must to pop in and treat yourself to a beautiful meal at one of Saint John's more charming restaurants, 'The Falls Restaurant'. You will get a healthy sized meal from $7-18 each, all while enjoying some fantastic views of the river (including the 'famous' Reversing Falls).
The Falls are definitely not worth flying to Saint John for, but if you're in the city for a few days, it is a must.
If you were looking for something else to complete your day, I suggest you visit the 'Irving Nature Park', but that's another story.