Flood, Ebb, and Slack: How Tidal Currents Work

How tidal currents work: flood and ebb direction, slack water at the turn, current speed in knots and set, and why slack water does not always line up with high and low tide.

Tidal currents are the horizontal movement of water that accompanies the vertical rise and fall of the tide. Where the tide height tells you how deep the water is, the current tells you which way it is flowing and how fast. Currents matter most in channels, inlets, and estuaries, where they can run at several knots.

Flood, ebb, and slack

• Flood current — water flowing inshore or up an estuary, generally while the tide is rising.
• Ebb current — water flowing seaward or down an estuary, generally while the tide is falling.
• Slack water — the brief pause when the current is near zero as it turns between flood and ebb.

Current speed is measured in knots (nautical miles per hour). Its direction, called the set, is given as a compass bearing — the direction the water is flowing toward.

Slack water is not always at high tide

A common assumption is that the current stops at high and low water. That is only true in some places. The timing depends on how the tide wave behaves locally. Where the tide acts like a progressive wave, as in a long open channel, the strongest flood and ebb occur near high and low water, and slack falls near the midpoint. Where it acts like a standing wave, as at the head of a closed bay, the reverse is true: the fastest current is near mid-tide and slack water occurs near high and low water. Local current tables are the reliable guide.

Reversing and rotary currents

In narrow channels the current is usually reversing: it runs one way on the flood, then turns and runs back on the ebb. In open coastal water the current is often rotary instead, changing direction continuously through the compass over a tidal cycle rather than reversing along a single line.