Flood, ebb, and slack water: how to read tidal currents

Flood, ebb, and slack describe horizontal water movement—not tide height. Flood current moves toward shore or up an estuary; ebb moves away from shore or down-estuary; slack is the near-minimum current around a reversal. High and low tide may happen near those events at some locations, but NOAA warns that the shortcut does not hold for most waterways.

The four phases of a reversing tidal current

Many inlets, rivers, and channels have a current that reverses along a dominant axis. One cycle contains four planning landmarks:

1. Slack before flood — ebb weakens toward its minimum before the direction turns landward or up-channel.
2. Maximum flood — the strongest predicted current in the flood direction.
3. Slack before ebb — flood weakens before the direction turns seaward or down-channel.
4. Maximum ebb — the strongest predicted current in the ebb direction.

“Slack” does not always mean perfectly motionless water. NOAA may use it for the minimum speed in a current that never reaches zero, and wind, river discharge, or circulation can keep water moving even while the tidal component is weak.

Why high tide and slack water can disagree

A tide station measures vertical rise and fall. A current station predicts horizontal speed and direction. In a standing-wave setting, slack can fall near high and low water. At a progressive-wave entrance, maximum current can occur near high or low water instead. In a hydraulic current—where two connected bodies of water stand at different heights—the greatest flow comes when that height difference is greatest.

That is why one generic tide chart cannot tell you the slack time in a canal or inlet. NOAA analyzes tide and current observations separately and publishes separate predictions. Use the tide chart for water level and the current prediction for flow.

How to read a max-and-slack table

The clearest NOAA current table lists a time and predicted speed for each maximum flood and maximum ebb, with slack-before-flood and slack-before-ebb between them. Read it in chronological order rather than treating one event as a day-long condition:

• Direction is turning after slack, strengthening toward a maximum, then weakening toward the next slack.
• Speed is usually reported in knots. A maximum is the peak of the predicted tidal component, not a promise that weather will leave it unchanged.
• The named flood and ebb directions belong to that station. Around a bend or across a wide channel, the direction at your exact position can differ.

Harmonic current stations can also provide interval curves. Subordinate current stations are limited to maximum and slack predictions because they are derived from offsets to a reference station. For planning a narrow transit, those named landmarks are often the useful answer.

Depth and station location matter

Current changes across a channel and through the water column. NOAA current stations can publish a selected depth, or bin, for a prediction. The station marker therefore represents a measured location and depth—not an entire bay. Check the station metadata before applying one prediction to a different side of a channel, a shallow flat, or a surface-drift question.

Prediction is the tidal component, not the whole current

NOAA current predictions are driven by the periodic tidal signal. Nontidal flow from wind, rivers, density differences, and ocean circulation can add to or subtract from it. A strong river can lengthen ebb and weaken flood; sustained wind can change the surface flow; a storm can make a normally manageable prediction irrelevant.

Use the prediction as the repeatable astronomical baseline. Then check observations, weather, marine forecasts, local notices, and conditions at the water before committing. Perigee exposes NOAA current tools through its REST API and MCP server, and the free boating planner combines tide, wind, daylight, and source freshness for a nearby tide station. It supports a decision; it does not replace a current table, chart, or navigation judgment.