A tidal bore is a natural event that occurs when the leading edge of an incoming tide forms a wave that travels up a river or narrow bay, reversing the water’s flow. This powerful, singular wave is a form of hydraulic jump that moves against the river’s downstream current. The phenomenon is a visible manifestation of the tide, where the gradual rise of water is compressed into a sudden, fast-moving wall. It only happens in specific locations where geography and tidal forces align to create this temporary, upstream surge.
The Physical Requirements for Bore Formation
The creation of a tidal bore depends on a precise alignment of physical conditions that work together to amplify the incoming tide. First, the coastal region must exhibit a macrotidal environment, meaning it experiences an exceptionally large difference between high and low tide, typically a range exceeding 4.5 to 6 meters (about 15 to 20 feet). This high tidal range provides the significant volume of water necessary to force the dramatic surge upstream.
The shape of the estuary and river channel is equally important, acting as a natural funnel to concentrate the tidal energy. The river mouth must be wide and taper gradually into a much narrower, shallow river channel, a geometry that forces the large volume of incoming water to compress laterally and vertically. As the tide moves from the deep, broad ocean into the shallow, constricted river, the wave’s speed decreases, causing the wave to steepen and break.
This process is further aided by a shallow riverbed with a gentle, consistent slope that allows the tidal wave to propagate far inland without dissipating its energy prematurely. The incoming flood tide must also overcome the resistance of the river’s freshwater flowing in the opposite direction. While a river’s outflow opposes the bore, the most substantial bores often occur during periods of low freshwater discharge, as a strong river current can suppress the tidal wave’s formation.
How Tidal Bores Look and Behave
The tidal bore manifests as a distinctive wave structure once the necessary conditions are met. It typically appears as a single, breaking wavefront, characterized by a turbulent roller of churning, aerated water, similar to a traveling hydraulic jump. Alternatively, it can take the form of an undular bore, which features a smooth, non-breaking lead wave followed by a series of smaller, secondary waves known as whelps or whelms.
These powerful waves move rapidly upriver, often traveling at speeds exceeding 25 kilometers per hour (about 15 miles per hour). The passage of the bore is preceded by a distinctive, powerful roar or rumble that can often be heard several kilometers away. This sound is generated by the intense turbulence, air bubbles trapped within the breaking wave, and the force of the water eroding the riverbed and striking obstacles.
A bore’s passage results in a rapid and substantial increase in water depth, known as the tidal jump, which can raise the water level by several meters in moments. Following the bore, the entire character of the river changes instantly, as the current abruptly reverses direction and flows strongly upstream. This sudden, turbulent force poses a hazard to navigation, but it has also created a unique opportunity for surfers, who can ride the continuously moving wave for long distances upstream.
Major Rivers and Bays Where Bores Are Found
Tidal bores occur in only a few dozen locations worldwide, but the most prominent examples share the specific geography of a wide, funnel-shaped estuary leading to a shallow river. The world’s largest and most famous tidal bore is found on the Qiantang River in China, where it is known as the Silver Dragon. Here, the bore can reach heights of up to 9 meters (nearly 30 feet) and travel at speeds of up to 40 kilometers per hour, attracting massive crowds during the peak lunar tides.
In the United Kingdom, the Severn Bore is a well-known example that forms in the River Severn estuary, which has one of the world’s largest tidal ranges, sometimes exceeding 15 meters. The bore on the Severn can form a wave up to two meters high, a size sufficient to allow surfers to ride it far inland towards the city of Gloucester. Similarly, the Amazon River in South America produces a bore locally called the Pororoca, which sweeps up the vast, gently sloped Amazon basin and its tributaries, forming a wave that can be several miles across and reach heights of up to 7.6 meters (25 feet).
In North America, the Turnagain Arm in Alaska, which is part of the Cook Inlet, is known for its bore. This location’s extreme tidal range and narrow, shallow channel create a powerful bore that regularly moves inland and is a distinctive feature of the region’s dynamic coastline.