Why tide times differ between nearby places

The tide travels, and the coast shapes it

The pull of the moon and sun sets the oceans in motion, but what reaches a particular beach is the result of that motion working its way across an ocean basin, onto the shallower continental shelf and into bays and channels. The tide arrives progressively, not everywhere at once. High water moves along the shore, so each place has its own time.

The speed of this wave depends on the depth of the water. In the deep ocean it moves very fast, and in shallow water it slows markedly. A tide crossing a wide, shallow bay therefore takes longer to reach the far end than the distance alone would suggest, and friction with the seabed alters its height along the way.

Basin shape: funnels and resonance

The shape of a bay or gulf can magnify the tide or mute it. Where a bay narrows and shallows towards its head, the incoming water is forced into a smaller space and the range grows. Some bays also have a natural period of oscillation, rather like water sloshing in a bath, that happens to be close to the period of the tide. The tide then reinforces itself, and bays of this kind have some of the largest tidal ranges in the world.

The opposite happens too. In parts of some seas the tide circulates around points where the range is close to zero and increases with distance from them. Coasts that are not far apart in ocean terms can therefore have very different ranges.

Inlets, lagoons and narrow entrances

A lagoon, harbour or sound connected to the sea by a narrow entrance cannot fill or empty quickly. Water starts pouring in as the sea rises outside, but the entrance limits how fast it can pass. By the time the sea outside has reached high water, the level inside is still catching up. High water inside therefore comes later than outside, and usually does not rise as high. The same delay applies on the way out.

The effect can be large. A beach on the open coast and a boat ramp just inside the inlet, a short walk apart, may differ substantially in both time and height. It is also the reason the current in an inlet often keeps flowing in after high water on the outside, a point taken up in the article on slack water.

Rivers and estuaries

The tide can travel a long way up a river, and it changes as it goes. High water arrives progressively later upstream. Low water is usually delayed even more, so the rise becomes shorter and steeper and the fall longer and slower. Farther up, the range shrinks until the tide fades out altogether.

River flow is added on top. After heavy rain or snowmelt, the water level in the upper estuary can sit well above the predicted tide, and the outgoing current can dominate. Predictions for river stations reflect ordinary river conditions and are least reliable when the river is far from ordinary.

Reference stations and secondary stations

Tide predictions come in two broad kinds. Reference stations, also called primary stations or standard ports depending on the country, are places where water levels have been recorded for a long time. Predictions for them are calculated directly from that record and are the most dependable.

Secondary stations, also called subordinate stations or secondary ports, are places with a shorter record. Their predictions are commonly derived by taking a reference station and applying corrections: a number of minutes to add to or subtract from the times, and a difference or ratio to apply to the heights. These corrections are averages. They work well on ordinary days and less well when the tide or the weather is unusual, so the results should be treated as somewhat less precise than those for a reference station.

Choosing a station, and borrowing one with care

The nearest station in a straight line is not always the right one. The right one is the station that shares the same water as the place of interest.

Prefer a station in the same body of water, on the same side of any inlet, narrows, bar or causeway.

For a spot up a river or inside a lagoon, use a station that is also inside, even if an open-coast station is closer on the map.

Where no station fits well, look at the stations on either side. If they agree closely, the spot between them probably behaves similarly. If they disagree, expect the truth to lie somewhere between and allow a wide margin.

Treat both the times and the heights from a borrowed station as approximate, and do not carry either across a constriction.

CatchWindow location pages show tide predictions where a tide station applies. It is worth bearing in mind that a prediction for a station some distance away describes that station, not the exact spot where a boat is launched or a line is cast. For navigation and for any decision where depth or timing is critical, use the official tide tables and charts, which list the stations and corrections for the area, and check them against what the water is actually doing on the day.