Lowwater

Lowwater refers to the minimum water level reached in a tidal cycle at a specific coastal location. In everyday language it corresponds to low tide, but in navigation, surveying, and law it is often tied to a chart datum such as mean low water (MLW), mean lower low water (MLLW), or lowest astronomical tide (LAT). The exact reference varies by charting authority and country.

Low water is determined by celestial and local factors. The Moon’s and Sun’s gravitational forces drive tides, while local bathymetry, coastline shape, wind, and atmospheric pressure modify the actual low-water height. As a result, low-water levels can differ from place to place and from year to year, even on the same calendar date.

In nautical charting and land mapping, the low-water line marks the point reached by the sea at low tide and is used to describe shorelines and intertidal zones. Some jurisdictions also designate a low-water mark for legal or planning purposes, which can influence property boundaries and coastal development rules.

Measurement and monitoring rely on tide gauges and standardized datums. Information about low water supports harbor operations, dredging schedules, flood and drought planning, and coastal engineering, as well as scientific research into long-term sea-level changes.