Tidal range is the vertical difference between the water level at high tide and the water level at low tide. If high water reaches 4 metres above the relevant reference level and low water reaches 1 metre, the tidal range is 3 metres.
That difference is not the same everywhere and it does not remain constant. Some coastlines experience relatively small changes in water level, while others expose and cover large areas of the shore as the tide moves between high and low water.
Understanding Tidal Range
Every tidal cycle moves between higher and lower water levels. Tidal range describes the size of that vertical change rather than the height of either tide on its own.
This makes it different from simply describing high tide and low tide. Those terms identify stages of the tidal cycle. Tidal range measures the vertical difference between them.
The underlying rise and fall of sea level is driven primarily by the gravitational influence of the Moon and Sun together with the motion of the Earth-Moon system. That wider mechanism is covered in How Do Tides Work?.
How Is Tidal Range Calculated?
Tidal range can be calculated by subtracting the height of low water from the height of high water for the relevant tidal cycle.
For example, if a tide table gives a high-water height of 4.5 metres and a low-water height of 1.2 metres, the tidal range is 3.3 metres.
- High water: 4.5 m
- Low water: 1.2 m
- Tidal range: 3.3 m
The reference level used for published tide heights depends on the tidal authority and location, so tide heights should always be interpreted using the datum or reference information supplied with the local tide data.
Large and Small Tidal Ranges
A large tidal range means there is a substantial vertical difference between high and low water. A small tidal range means the change in water level is much smaller.
The visible effect depends strongly on the shape of the coastline. On a gently sloping beach, even a moderate vertical change can move the shoreline a considerable horizontal distance. Along a steep cliff or harbour wall, the same vertical change may produce much less horizontal movement.
This is why tidal range and the amount of beach exposed are related but not interchangeable. The slope and shape of the coast determine how a vertical change in water level translates into a changing shoreline.
Why Tidal Range Changes Through the Month
Tidal range expands and contracts through the lunar month as the relative positions of the Sun, Moon and Earth change.
Around new moon and full moon, the tidal influences of the Moon and Sun reinforce each other. This produces spring tides, when high tides tend to be higher and low tides lower than average. The resulting tidal range is generally larger.
Around the first and third quarter moon, the tidal influences of the Sun and Moon partially counteract each other. These are neap tides, when the difference between high and low water is generally smaller.
This repeating variation is explained in more detail in Spring Tide vs Neap Tide.
Why Tidal Range Varies Between Coastlines
Two coastlines can experience very different tidal ranges even though both are influenced by the same Moon and Sun. The reason lies in the way tidal waves interact with ocean basins and coastal geography.
The shape and depth of an ocean basin, continental shelves, bays, channels and surrounding land can alter tidal motion. Resonance within an ocean basin or bay can amplify the tidal response, while the geometry and depth of coastal waters can further modify how the tide appears at the shoreline.
This means latitude alone does not determine tidal range. Local and regional geography can be far more important in determining how strongly the tide appears at a particular coast.
When Geography Amplifies the Tide
Certain ocean basins and coastal shapes can strongly amplify tidal movement. The Bay of Fundy in Canada is one of the clearest examples, with tidal ranges that can reach around 16 metres in parts of the bay.
Its exceptional tides are not produced by one factor alone. The dimensions of the Bay of Fundy and Gulf of Maine system allow the incoming Atlantic tidal motion to interact with the basin close to its natural resonant period. The narrowing and shallowing geometry toward parts of the upper bay further influences how that tidal energy appears along the coast.
This illustrates an important principle: astronomical forces generate the tides, but the shape, depth and dimensions of the ocean basin and coastline strongly influence the tidal range experienced at a specific place.
Tidal Range and the Shape of the Coast
Tidal range has a direct effect on how much of the intertidal coast moves between being submerged and exposed.
Where the range is large, beaches can become substantially wider at low water, rocks and reefs may emerge, tidal flats can extend far from the high-water shoreline and channels can change significantly in depth.
At high water, many of those features may disappear beneath the surface again. Where tidal range is small, the transition can be much less dramatic.
This becomes especially relevant around coastal environments where beaches, channels, river mouths and shallow seabed features interact. At Mundaka, for example, the coastal setting around the estuary provides useful context for understanding why changing water levels can alter the physical character of a coastal environment through the tidal cycle.
The same coastline can therefore look and function differently depending on both the current stage of the tide and the size of the tidal range during that part of the spring-neap cycle.
Tidal Range and Tidal Currents
Tidal range describes a vertical difference in water level. Tidal currents describe horizontal movement of water. The two concepts are connected, but they are not the same measurement.
As tides rise and fall, water often has to move into or out of bays, estuaries, channels and other coastal areas. Local geography can concentrate that movement and produce strong tidal currents.
A large tidal range can contribute to substantial tidal movement in some locations, but tidal range alone does not tell you the speed or direction of a current. The shape and depth of the surrounding waterway, the timing of the tide and other local conditions are also important.
Tidal currents are one part of the wider circulation of seawater explored in What Causes Ocean Currents?.
Reading Tidal Range in a Tide Table
A tide table normally provides predicted times and heights for high and low water at a particular reference location. Comparing the predicted high and low heights gives the tidal range for that part of the cycle.
The figures are location-specific. Tide timing and height can differ between nearby places, and predictions from a reference port or tidal station may not perfectly represent conditions at another beach or section of coast.
Weather can also influence the water level actually observed. Wind and atmospheric pressure can raise or lower coastal water levels relative to the astronomical tide, so a tide prediction should not be treated as a complete description of future sea conditions.
A Measure of How Much the Coast Changes
Tidal range is a simple measurement with a large physical effect. It tells us how far sea level moves vertically between high and low water, but that vertical difference can transform an entire stretch of coastline.
Its size changes through the lunar cycle and varies enormously between locations because tides interact with the shape, depth and scale of ocean basins and coastlines.
Understanding tidal range therefore adds another layer to reading the coast. High and low tide tell you where the tidal cycle is. Tidal range tells you how large the difference between those two states actually is.