Why the world's most extreme tides are trapped in specific coastal funnels

Nature · Published 3 October 2026 Swipe more like this →

In short: Tidal range is the height difference between high and low tide, governed by the gravitational pull of the Moon and Sun. While the open ocean sees a modest one-metre shift, coastal geography can amplify this significantly. Some regions experience massive surges, while others remain nearly still, creating a global tidal puzzle.

At its core, the tidal range is a predictable consequence of celestial mechanics. The rise and fall of sea levels are driven by the gravitational forces of the Moon and Sun, combined with the rotation of the Earth and the centrifugal force of our planet's progression around the Earth-Moon barycenter. When the Moon and Sun align at syzygy, their forces reinforce one another, resulting in the larger spring tides. Conversely, when these forces act at right angles during neap tides, the range between high and low water reaches its minimum.

However, the actual experience of a tide at any given coastline is heavily dictated by local geography. While the open ocean maintains a typical range of about one metre, coastal basins can act as funnels, amplifying the incoming water. This explains why the world's most dramatic tidal ranges are found in specific locations like the Bay of Fundy in Canada, which holds the record for the largest mean range at 11.7 metres, and the Bristol Channel, which reaches 9.60 metres. These areas demonstrate how the volume of water and the shape of the basin dictate the intensity of the tide.

It is important to distinguish between predictable astronomical cycles and unpredictable environmental variables. While hydrographic offices can map tides with high precision based on celestial patterns, weather events introduce significant volatility. Sustained storm-force winds and low barometric pressure can push water levels well beyond predicted values, particularly in narrow bays, leading to localized flooding. These meteorological influences are inherently unpredictable, meaning that even in regions with well-understood tidal patterns, the sea can occasionally defy the charts.

Scientists categorize these coastal systems based on their mean tidal range. Systems with a range below two metres are termed micro-tidal, while those between two and four metres are meso-tidal. Macro-tidal systems exceed four metres, and a select few exceeding six metres are classified as hypertidal. At the opposite end of the spectrum, some regions like the Mediterranean, Baltic, and Caribbean Seas experience minimal tidal movement, sometimes centering on an amphidromic point where the range is almost zero.

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