Vertical and horizontal

High water is not
slack water.

A tide table tells you how high the water will be. It does not tell you which way it is running, or how hard. At an entrance or a bar, the second question is the one that hurts you.

01 · The distinction

The tide rises. The tidal stream runs.

Two separate things are usually both called "the tide":

The tide proper: the vertical rise and fall of the water surface. This is what a tide table predicts, in metres above a datum.
The tidal stream or tidal current: the horizontal flow of water produced as that rise and fall moves water in and out of a bay, estuary or channel. Measured in knots, with a direction.

They are caused by the same thing and they are not in step. Knowing the height at 10:00 tells you nothing directly about whether the water is running in, running out, or still.

For a swimmer, a diver, a kayaker or anyone crossing an entrance, the horizontal flow is usually the more important of the two, and it is the one almost nobody publishes.

02 · Timing

When does the current actually turn?

The intuitive assumption is that the current stops at high water and low water. Sometimes that is close to true, and often it is not.

It depends on how the tide wave behaves in that particular body of water. There are two idealised cases, and real places sit somewhere between them:

A standing wave

In a basin where the tide wave reflects back on itself, the flow is fastest around mid-tide and slack occurs near high and low water. Here the intuitive rule is roughly right.

A progressive wave

Where the tide wave travels through, as in a long channel or a river, the flow is fastest around high and low water, and slack occurs near mid-tide. Here the intuitive rule is exactly backwards.

In real estuaries, especially where a river discharges, the picture is further distorted: the outgoing flow often runs longer and harder than the incoming one, and slack water can lag high water by a substantial part of an hour or more. In a river in flood, the ebb may barely stop at all.

The practical conclusion: at a specific entrance, slack water is a local fact that has to be known or looked up. It cannot be derived from the height table, and local knowledge or an official tidal stream prediction is the only reliable source.

03 · Availability

Why tidal current predictions are so much rarer than tide tables

You can find a tide height table for hundreds of Australian locations. Tidal stream predictions exist for a small fraction of that. There are two good reasons.

First, currents are more variable and less dominated by the tide. Australia's Integrated Marine Observing System puts it directly, contrasting currents with sea level:

"For sea level, in contrast, there are few places where the amplitude of non-tidal variability is comparable to, let alone exceeds, the amplitude of tidal variability."
IMOS OceanCurrent

In other words: sea level is mostly tide, so a harmonic prediction gets you most of the answer. Currents frequently are not mostly tide, because wind, river flow and ocean circulation contribute as much or more. A purely tidal prediction of current is therefore a much weaker product than a purely tidal prediction of height.

Second, currents are harder to measure and to model. A tide gauge is a cheap, robust instrument that can sit at a wharf for decades. Measuring the flow through a channel requires current meters deployed in the flow. Published comparisons of ocean models against Australian observations have found the models do notably worse on velocity than on height.

So the scarcity is not an oversight. Current is genuinely a harder quantity, and the authorities publish it only where they have the observations to support it.

04 · Safety

Bars, ramps and channels: where the difference is dangerous

Coastal bar crossings are the clearest case where reading the height and ignoring the stream gets people hurt.

The mechanism is straightforward. On an ebb tide, water runs out of the entrance against incoming swell. Opposing current and waves make the waves shorter, steeper and more likely to break. The same swell that is unremarkable outside can become a breaking sea on the bar, and the change can happen over the course of an hour.

Other places the stream matters more than the height:

Narrow entrances and headlands, where flow accelerates and can exceed a boat's comfortable speed.
Boat ramps in a channel, where a cross-current on retrieval is a bigger problem than depth.
Rock fishing platforms, where the concern is a rising tide and the state of the sea rather than current, but where the same "the table said it was low" reasoning causes trouble.
Swimming and drifting near entrances on a running tide.

What to do instead of estimating: for any bar crossing, use the official advice for that bar. Maritime Safety Queensland and the equivalent state authorities publish bar crossing guidance and, for many bars, live camera or condition reports. Log on with your local marine rescue group. Local knowledge about when a specific bar is workable is not something a national tide site can or should replace, and we are not going to pretend otherwise.

05 · Our own data

What Atmosphr's ocean panel shows, and what it does not

The ocean conditions panel in the weather app includes a modelled current. It is worth being precise about what that is.

It comes from a global numerical ocean model on a grid of roughly eight kilometres. That means:

It is a total modelled current, including wind-driven and ocean circulation components, not a tidal stream prediction.
It cannot resolve an entrance, a bar, a channel or a river mouth. Those features are smaller than a single grid cell. A model cell covering a bar is really telling you about the open water nearby.
It is guidance for the general offshore picture, and the panel says as much.

The panel carries this, and it means what it says:

Planning guidance only. Coastal tide and current accuracy is limited. Never use this panel for navigation or safety decisions.

If you take one thing from this page: a modelled current at eight kilometre resolution and the flow through your local entrance are not the same quantity, and treating them as interchangeable is the exact error this page exists to prevent. More on modelled versus predicted.

06 · Where to look

Official tidal stream information

Australian National Tide Tables from the Australian Hydrographic Office include tidal stream information for locations where it is published. This is the authoritative source.
Bureau of Meteorology tidal stream predictions exist for a small number of significant passages. Port Phillip Heads, known as The Rip, is one of the better known Australian examples and appears alongside the Victorian tide tables.
Nautical charts carry tidal stream diamonds with tabulated rates and directions relative to high water at a reference port. If you have the chart, you have the data.
State maritime authorities and port authorities publish local guidance for their waters, including bar crossing advice.
Local marine rescue groups, for current conditions on a specific bar, which is information no published table contains.