Tidal currents: flood, ebb and slack water

Tides don’t just go up and down

A tide table shows heights: the water rises to high tide and falls to low tide. But for the level to rise inside a bay or a harbor, water has to flow in from outside, and for it to fall, water has to flow back out. That back-and-forth flow is the tidal current.

Out in the open ocean, far from land, tidal currents are usually weak. Near the coast, the water has to squeeze through narrow or shallow places, and it speeds up there, the same way a river speeds up where its banks close in. In an inlet, a channel between islands or a strait, the current can reach several knots.

A knot is the standard unit of speed on the water: one nautical mile per hour, about 1.15 mph. That may not sound like much, but a 2-knot current is already faster than many people can swim.

Flood, ebb and slack

Each tide cycle has three phases for the current:

• Flood current: water flows in toward the coast, up bays, rivers and sounds, while the tide rises.
• Ebb current: water flows back out to sea while the tide falls.
• Slack water: the short lull when the current stops or nearly stops before it reverses.

In inlets, channels and tidal rivers, the current runs back and forth along the same path, like traffic in a hallway: in, out, in. These are called reversing currents. Offshore, something else usually happens. The current doesn’t stop; it slowly swings around the compass over the 12-hour-plus cycle, like the hand of a clock. These rotary currents are usually weaker.

When currents run hardest: the rule of twelfths

In many bays and estuaries, the current is weak near high and low tide and strongest around mid-tide. A rule of thumb sailors use, the rule of twelfths, shows why. On a semidiurnal tide that takes about six hours to rise, the water climbs:

Hour of the rising tide	Share of the total rise
1st	1/12
2nd	2/12
3rd	3/12
4th	3/12
5th	2/12
6th	1/12

Here’s an example with numbers. In a Georgia or South Carolina tidal creek, on a day with a 6-foot range between low and high tide, the water rises about 6 inches in the first hour, 1 foot in the second, 1.5 feet in the third, another 1.5 feet in the fourth, 1 foot in the fifth and 6 inches in the sixth. Raising the level a foot and a half in an hour takes far more water than raising it 6 inches, so the flood runs hardest in the third and fourth hours.

The rule is approximate and doesn’t fit every place equally well, but the idea holds: for calm water, aim for the hours around the turn of the tide; for moving water, aim for the middle hours.

Slack water doesn’t always come at high tide

This is the most common mistake: assuming the current turns exactly at high or low tide. Sometimes it does, often it doesn’t.

In a long bay or tidal river, the tide travels as a wave. When it’s already high tide at the inlet, the water farther up is still rising, so water keeps pouring in to fill it. At the inlet, the flood can keep running well after high tide. The same thing happens in reverse on the ebb.

In channels that connect two bodies of water with different tides, the current depends on the difference in water level between the two ends, not on the tide at one end. That’s what drives the strong currents in the Cape Cod Canal and in New York’s Hell Gate on the East River: the tide reaches each end at a different time and a different height, and the water rushes from one side to the other.

That’s why NOAA publishes tidal current predictions separately from tide predictions. For many locations, they give the times of slack water and the time, direction and speed of maximum flood and ebb. Where there’s no current prediction, the tide table is a good starting point, but watch the water too: buoys, anchored boats, crab pot floats and drifting weed all show which way it’s running.

Where currents are strongest

Tidal currents speed up wherever the water has less room to pass:

• inlets and harbor entrances, such as the inlets of the Outer Banks, the rest of the Carolinas and Florida’s Atlantic coast;
• straits and narrow channels, like the Golden Gate at the mouth of San Francisco Bay;
• points and headlands, where the current wraps around the land and spins eddies behind it;
• sandbars at river mouths and inlets, where waves also break.

Spring and neap tides matter here too. Around the new and full moon, more water moves in and out in the same time, so currents run harder. Around the quarter moons, they slack off. Our guide to spring and neap tides explains the cycle.

Wind against tide

Wind changes the picture. When it blows against the current, it stacks up a short, steep, confused chop, far worse than the same wind would raise over still water. At an inlet bar, an onshore wind against a strong ebb is a combination small boats and kayaks should avoid. Inlet conditions are often at their worst then, on a strong ebb, when incoming waves stand up and break over the bar.

The opposite also holds: wind with the current flattens the sea but makes the boat drift faster, which matters if you’re anchoring or drift fishing.

Rip currents are a different thing

At the beach, people often mix up rip currents and tidal currents, but they aren’t the same. A rip current is a narrow channel of water flowing back out to sea after waves push it toward the shore. Waves are the engine.

The tide still affects them. At many beaches, a falling tide exposes sandbars with gaps between them, and rips pour out through those gaps more strongly. If you get caught in one, don’t fight it: swim parallel to the shore until you’re out of the current, then head back in, or float and signal for help.

How to make currents work for you

• For fishing, moving water usually beats slack: it carries bait and gets fish feeding. We cover it in fishing the tides.
• For diving or swimming near an inlet, slack water is the safest window. Without a current prediction, expect it to come a little after high or low tide.
• By kayak or small boat, either head out against the current while you’re fresh and let it carry you home, or ride it out and time your return for after it turns, so it’s behind you.
• For wading or clamming on flats that flood, remember that the water rises fastest in the middle hours. There’s more in our guide to tide safety.