A tidal stream is the periodic movement of water in a horizontal direction that is due ultimately to the same astronomical causes as the tide (whereas the tide is a movement in the vertical direction).
The tidal stream associated with a rising tide is called the flood stream; that associated with a falling tide is called the ebb stream. A tidal stream is often described by the direction to which it runs. There is usually a period when the flow ceases, known as slack water, before the stream changes direction. Interestingly, slack water does not necessarily occur at times of high or low water.
Tidal streams can be analysed and predicted in much the same way as tides. It is generally simpler however, to compare the stream with the tide at a nearby Standard Port and determine the time difference between high or low water at the port and the beginning of the ebb or flood stream in a particular area.
This is the method used to compute the tidal stream predictions for Tory Channel / Kura te Au and Te Aumiti / French Pass published on this website.