MLLW

Mean Lower Low Water (MLLW) is a tidal datum used as a reference for measuring depths on nautical charts. It represents the mean value of the lower of the two daily low tides, averaged over a 19-year Metonic cycle. Because tides vary with the lunar cycle and local coastlines, MLLW provides a stable, location-specific reference level for determining water depth at low tide.

Calculation and scope: At a given tide station, the lower low water height is identified for each tidal day. These values are collected over 19 years and the arithmetic mean of the lower low water heights is taken as MLLW. The datum is anchored to a specific geographic location and tide gauge, and may differ from MLLW at nearby sites due to local bathymetry, shoreline changes, and gauge recording periods.

Usage and significance: In the United States, depths on standard nautical charts are typically referenced to MLLW, making it a critical datum for navigation, dredging, and harbor planning. MLLW is distinct from mean sea level and from other datums such as Mean High Water (MHW) or Mean Lower Water at other locations. Because it can be affected by long-term sea level rise and vertical land movement, chart datums like MLLW are periodically updated to reflect changes in the coastal environment.

Related terms: Other tidal datums include Mean High Water (MHW), Mean Sea Level (MSL), and Lowest Low Water (LLW). Some regions may also reference datums such as Lowest Astronomical Tide (LAT) for specialized applications.