Tidal range versus tidal current

Up and down versus to and fro

As the tide rises on a coast, water has to come from somewhere, and as it falls, water has to go somewhere. That movement is the tidal current, often called the tidal stream. The current that accompanies the rising tide is the flood, and the current that accompanies the falling tide is the ebb. Strictly, flood and ebb describe the flow, while rising and falling describe the level.

In enclosed waters such as channels and estuaries, the flow mostly runs back and forth along the channel, with a pause called slack water at each turn. The turn does not necessarily happen at high or low water, as the article on slack water explains.

Why a big range does not guarantee a fast current

The speed of a tidal current depends on how much water must pass a given point in the time available, and on how much room it has to pass through. Range is only one ingredient in the first of those.

Picture a straight, open coast with a large tidal range. The level rises several metres, but the sea is wide and deep, and the water needed to raise it arrives across a broad front. The current along the beach may be modest. Now picture a large lagoon with a small range, connected to the sea by a single narrow entrance. Raising the whole lagoon by even half a metre (about 1.6 ft) takes an enormous volume of water, and all of it must pass in through that one gap and out again on every tide. The current in the entrance can be strong even though the range is unimpressive.

How constrictions speed the water up

Water moving through a channel behaves much like water in a hose with a thumb over the end. When the same volume has to pass through a smaller opening, it moves faster. Any feature that reduces the cross-section available to the flow has this effect.

Narrows and passes between islands, or between an island and the mainland.

Harbour and lagoon entrances, especially those narrowed by breakwaters or sandbars.

Headlands, where the flow along a coast is squeezed as it bends around the point.

Shoals and underwater ridges, where the channel becomes shallower without becoming narrower.

Fast water meeting an obstruction or an uneven bottom also becomes turbulent. Downstream of a headland or a pass there are often eddies, where water circles back against the main flow, and patches of steep, breaking water known as tide rips or overfalls. Eddies can be useful to a small boat working against the current. Overfalls are best avoided.

Set and drift in plain terms

Navigators describe a current with two words. Set is the direction the current is flowing towards. Drift is its speed. A current with a set of east and a drift of 2 knots is carrying everything in it eastwards at about 3.7 km/h (2.3 mph). Some sources use the word rate for the speed and keep drift for the distance a vessel is carried over a period of time. The idea is the same.

Note the convention: a current is named for where it is going, while a wind is named for where it comes from. An easterly current and an easterly wind therefore move in opposite directions.

The practical meaning is simple. A boat's movement over the ground is its own movement through the water plus the movement of the water itself. A kayak paddled at 3 knots (about 5.6 km/h or 3.5 mph) straight into a 2-knot current makes only 1 knot of real progress. With the same current behind it, it makes 5. A current from the side pushes the boat sideways, so it has to be pointed upstream of its destination in order to arrive there.

What the two have in common

At any one place, range and current do rise and fall together through the fortnightly cycle. Around spring tides, when the range is largest, more water moves and the currents are stronger. Around neaps, both ease. The day's tidal range is therefore a fair guide to whether the current at a familiar spot will be stronger or weaker than usual. It is not a guide to how strong that current is in absolute terms, and it says nothing reliable about a place that has not been checked.

Using both when planning

Use each source for what it measures. Tide tables answer questions about height and depth: whether there will be enough water over a bar or at a ramp, how much beach will be exposed, how much anchor line to let out. Current tables, tidal stream atlases and the current information on charts answer questions about flow: when it turns, which way it runs and how hard.

CatchWindow location pages show a tidal-range figure where a tide station applies and current and slack-water information where there is a current station, and the map has a current layer for a broader view. These help with choosing a day and a time. They are not a substitute for official current tables, charts and marine forecasts, which are the proper basis for navigation, particularly through narrows or in unfamiliar water.