Which tidal datum is the average of the lower of two tides during a tidal cycle?

• A. Mean High Water
• B. Mean Water Level
• C. Mean Lower Water
• D. Mean Low Water

Answer: (D)

In a tidal day with two low tides, the lower of the two is called the lower low water. The datum that represents the long-term average of that specific lower low water is Mean Lower Low Water. That’s why it’s the best choice here: it directly corresponds to the average level of the lower low tide across many cycles, giving the reference for the lowest regular water level.

Mean High Water is the average of the higher high tides, not related to the lower low tide. Mean Water Level is a general reference for sea level, not the specific low-tide datum. Mean Low Water is the average of low waters overall, not the average of the lower of the two low tides.