On the face of it, a high tide comes in, reaches its highest and then begins to go out, so surely there's a point where they 'stop' as they reverse direction, whether that's transitioning from a high tide to a low tide, or in reverse, a low tide into a high. Although, logically, this makes sense, the reality isn't quite as black and white as this.
What is slack water?
When we consider the argument that tides must stop, even for the shortest of times, what is the name of that transition period? The answer to this is slack water. Slack water is when the water seems momentarily halted during the transition from high to low water (or reverse). It slows down significantly while this transition happens, but it doesn't truly stop.
The effects of slack water largely depend on where you are. If you're out in the middle of the ocean, you wouldn't even notice slack water or the changing tides at all. With no point of reference other than what's in the sky, it's hard to know what's actually happening.
Closer to shore, slack water is a lot more obvious. In bays and estuaries, slack water is a lot more noticeable as the tides reverse direction, but they're never truly stopping. Here's why.
The tides never truly stop
You probably know the tides are caused mainly by the moon's gravitational pull on our seas and oceans. Although to a lesser extent, the gravitational pull of the sun also affects this. When the sun and moon are aligned on the same side of the Earth, their gravitational pulls combine, causing a spring tide. When they're at right angles to one another, this causes a neap tide. You can read about these here.
The point is, the earth's orbit around the sun isn't fixed, nor is the moon's orbit fixed around the earth, meaning that during slack water, there are gravitational pulls in more than one area at any given time. One may have a horizontal pull, the other a vertical pull when looking from above, so even during slack water, the tides are constantly shifting in different directions.
Weather also affects the tides
We're starting to see why tides never truly stop, but it doesn't end with gravitational pulls of celestial objects; the weather plays a big part in tidal movements.
Strong winds can cause a storm surge, either pushing water further inland or holding water back, delaying the expected slack water period. What's more, high-pressure systems suppress tidal movements, while low-pressure systems (like during storms) can increase water levels, affecting the transition between high and low tides.
Finally, prolonged winds in one direction can cause wind-driven currents that oppose or enhance tidal movements, affecting the perceived stillness of tides.
The effects of topology on tidal movement
The lay of the land around or under the water has a profound effect on tidal movement, also contributing to the fact that tides never truly stop. Tides can be delayed due to the time required for water to funnel through narrow spaces such as estuaries, straits and inlets. These delays, combined with weather and celestial movement, mean that such areas are more susceptible to tidal movement than you might think.
But it doesn't stop there. In shallow coastal areas, the friction with the seabed slows water movement, affecting the timing and nature of slack water. On the reverse, in deepwater coastlines, these areas experience smoother tidal transitions with less noticeable slack water.
Finally, amphidromic points are areas in the ocean that have little to no perceived tidal movement due to wave interference, but this doesn’t mean the tides "stop"; rather, the water level remains relatively stable.
Extreme cases where the tidal movements appear stagnant
There are areas where tidal movements appear to be completely stagnant. Seiches (Standing Waves) tend to occur in semi-enclosed bays and lakes. Tidal motion can be overridden by oscillating water movements, causing water to appear still or erratic, but again, never truly stopped.
In places like the Amazon River, Tide-dominated river mouths see regular tidal bores that can create rapid water movement, preventing the perception of a tide pause.
Some regions, like the Gulf of Mexico, experience a weak tidal range, making slack water less noticeable. But again, you guessed it, the tides are not truly stopped.
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