A unique phenomenon created by the collision of the Bay of Fundy and the Saint John River. At low tide the river empties into the bay causing a series of rapids and whirlpools. As tides rise they slow the river current for a brief period called slack tide. The Bay's tides continue to rise, gradually reversing the flow of the river; rapids form again, peaking at high tide. A 8.5-metre (28-foot) tide cycle is roughly 12.5 hours.