A tidal stream (also called tidal current) is the horizontal movement of water caused by the tide rising and falling — distinct from tidal range, which describes the vertical change in water level.
Tidal Stream vs Tidal Range
- Tidal range — the vertical difference between high and low water at a location
- Tidal stream — the horizontal flow of water associated with the tide moving in and out
Both matter for navigation, but for different reasons — tidal range affects available depth, while tidal stream affects a vessel’s speed and course over ground.
How Tidal Streams Affect Navigation
A tidal stream running with a vessel’s course increases speed over ground; running against it reduces speed over ground and can significantly extend passage time. In narrow channels or restricted waters, a strong tidal stream can also push a vessel off its intended track if not properly accounted for in course calculations.
Tidal Stream Atlases and Tables
Many charts and specialized tidal stream atlases provide predicted tidal stream direction and rate for specific locations at different points in the tidal cycle, allowing officers to plan passages that take advantage of favorable streams and avoid unfavorable ones where practical.
Slack Water
Slack water is the brief period when the tidal stream reverses direction — momentarily reduced to near-zero flow — often the preferred time for certain operations (some pilotage, diving operations, or maneuvering in strong-current areas) where minimal current effect is desirable.
Exam Relevance
Tidal stream calculations, their effect on course and speed over ground, and passage planning around tidal streams are core Navigation topics in deck officer competency exams.