Mean Low Water (MLW)

Mean Low Water (MLW) is the arithmetic mean of all low-water heights observed at a station over the 19-year National Tidal Datum Epoch. It averages both lows of each tidal day in a semidiurnal regime and lies above Mean Lower Low Water where diurnal inequality is present. MLW served as the historic charting datum on US east-coast charts before the shift to Mean Lower Low Water, and it still defines the lower foreshore in several legal frameworks. The 19-year span removes the 18.61-year nodal modulation, keeping the datum stable across the epoch.