Tides rise and fall because of the waters movement due to gravity. As a tide rises, water moves toward shore. A rising tide, or incoming tide, is known as a flood tide. As a tide falls, water flows seaward. A falling, or outgoing, tide is known as an ebb tide. When the tide floods and ebbs, the movement of water shoreward or seaward can be significant or hardly noticeable. The horizontal movement of water caused by tide changes is often referred to as the tidal current. In estuaries, the tidal currents are often quite fast, exceeding 5 knots; in the open ocean, tidal currents rarely exceed 1 knot.

There is a period of time between an ebb tide and a flood tide when there is no appreciable horizontal movement of water. This period is known as "slack water."

There is another period of time between an ebb tide and a flood tide when there is no appreciable vertical movement of water. This period is known as a "stand."  

It would appear that the "slack water" period and the "stand" period should coincide, but this is not always the case. Along a regular coastline, the two periods should coincide. But where a larger bay connects with an ocean through a narrow channel, the tide may continue to "flood" in the channel long after the high water stand, and vice versa. In other words, the tidal current continues in the channel after the water has stopped rising or falling.

For stations well exposed to the ocean there is usually little difference between the time of high and low water and the beginning of the flood or ebb tide. However, in narrow channels, land-locked harbors, or on tidal rivers, the time of slack water may differ by 2 or 3 hours from the time of high- or low-water stand.

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