Definition
In a strait or canal joining two bodies of water whose tides differ in time or range, the difference in head drives a flow largely independent of the local vertical tide. Slack water occurs when the two levels equalise, not at local high or low water, so timings must be taken from a stream table rather than inferred from a tide table. Hell Gate, the Cape Cod Canal and parts of the Pentland Firth behave this way.
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