The constant rise and fall of the ocean’s surface, known as the tide, is governed primarily by the gravitational forces of the Moon and, to a lesser extent, the Sun. These astronomical influences create massive bulges of water, which we observe as high tides as the Earth rotates beneath them. The retreat of the water body results in low tides, completing a natural cycle that governs coastal environments globally. The intensity and range of this cycle are not static, as the alignment of the Earth and its celestial neighbors causes significant variations in water level extremes. The deepest low water marks are formally recognized by a name that reflects the powerful forces that create them.
Defining the Extreme Lows
The deepest low tides are scientifically categorized as the low water phase of “Spring Tides.” This term does not relate to the season but references the water “springing forth” or surging with greater force and magnitude. Spring Tides are characterized by the maximum difference between the high and low water levels observed in a given location.
When the high tide reaches its maximum height during this phase, the corresponding low tide retreats to its lowest possible point, exposing areas of the seafloor that are typically submerged. This substantial range is a distinguishing feature of the Spring Tide cycle, which occurs twice monthly in most coastal areas. The extreme low water mark during a Spring Tide exposes vast tidal flats and allows access to areas usually covered by the ocean.
In contrast, other periods experience less dramatic tidal variations, which are known as Neap Tides. During Neap Tides, the gravitational forces work against each other, resulting in a reduced tidal range and less extreme high and low water marks.
The Celestial Mechanics Behind Extreme Tides
The power behind the Spring Tide extremes is a specific astronomical arrangement known as syzygy, where the Sun, the Earth, and the Moon align in a nearly straight line in space. This alignment happens twice during the lunar cycle, coinciding with both the New Moon and the Full Moon phases. During the New Moon, the Moon is positioned between the Earth and the Sun, and during the Full Moon, the Earth is positioned between the Moon and the Sun.
In this straight-line configuration, the gravitational pull from the Sun and the Moon are combined and amplified, working in unison to maximize the tidal force exerted on the Earth’s oceans. The Moon is the primary driver of tidal forces because of its proximity, but the Sun’s contribution reinforces the lunar pull during syzygy. This reinforcement results in a much larger tidal bulge than usual, which translates into the highest possible high tides.
The conservation of water mass dictates that if the high tide is exceptionally high, the corresponding low tide must be exceptionally low. This exaggerated withdrawal of water from the shore is the direct mechanical cause of the deepest low tides that characterize the Spring Tide period.
How Low Tides Are Measured
To standardize navigation and charting, oceanographers and coastal authorities use a specific reference height, known as a tidal datum, to measure and predict water depths. The most common standard for measuring the lowest part of the tidal cycle is called Mean Lower Low Water (MLLW), especially in the United States. This datum is calculated by averaging the height of the lowest tide recorded each day over a specific nineteen-year period.
The MLLW level is assigned a value of zero on nautical charts, serving as the official baseline for depth measurements in coastal and harbor areas. Any water depth recorded on a chart is the distance from the seafloor up to this zero plane, ensuring mariners always have a conservative estimate of the minimum water depth.
The deepest low tides, those associated with the Spring Tide cycle, are defined as any water level that falls significantly below this established MLLW zero datum. These extremely low water events are recorded as negative values relative to MLLW on tidal prediction tables.