Academic Reading — Test 12

Reading passage

On the eastern seaboard of Canada, between the provinces of New Brunswick and Nova Scotia, lies a body of water renowned among oceanographers and visitors alike. The Bay of Fundy experiences the greatest tidal range recorded anywhere on Earth. At its head, in regions such as the Minas Basin, the difference between high and low water can exceed sixteen metres, a height comparable to a four-storey building. Each tidal cycle moves an astonishing volume of seawater into and out of the bay, an amount estimated to rival the combined flow of every river on the planet. To understand why such an extraordinary phenomenon occurs here, and not in countless other coastal inlets of similar size, one must look closely at two interacting factors: the physical shape of the bay and a property of moving water known as resonance. The tides themselves are produced by the gravitational pull of the Moon and, to a lesser degree, the Sun, acting upon the world's oceans. In the open sea, far from any landmass, the resulting rise and fall of the water surface is modest, typically less than a metre. It is only when these tidal waves encounter the continental shelf and are funnelled into bays and estuaries that their range can be dramatically amplified. The Bay of Fundy is roughly funnel-shaped: it is wide and comparatively deep at its mouth, where it opens into the Gulf of Maine, but it narrows steadily and grows shallower towards its inner reaches. As the incoming tidal wave is squeezed into this progressively constricting channel, the same quantity of water is forced into an ever-smaller cross-section. The water has nowhere to go but upwards, and so the tidal range increases as one travels inland. Geometry alone, however, cannot fully account for the sheer magnitude of the Fundy tides. The decisive factor is resonance, a phenomenon familiar from many areas of physics. Any enclosed or semi-enclosed body of water has a natural period of oscillation, the time it takes for a disturbance to travel from one end to the other and back again. This sloshing motion is sometimes likened to the way water rocks back and forth in a bathtub when it is disturbed. The natural period of oscillation for the Bay of Fundy, taken together with the adjoining Gulf of Maine, is close to thirteen hours. Remarkably, this is very near the period of the principal Atlantic tide, which arrives roughly every twelve hours and twenty-five minutes. Because the two periods are so closely matched, each new tidal pulse arrives at almost exactly the moment to reinforce the water already oscillating within the bay, much as a child on a swing is pushed higher by a series of well-timed shoves. This near-coincidence produces a powerful resonant effect that builds the tide to its remarkable height. Researchers have determined that the resonance in this region is not perfectly tuned; the match between the natural period and the tidal period is close but not exact. This subtle mismatch has practical consequences. Over geological timescales, the configuration of the basin can shift, and some scientists have suggested that the bay is gradually approaching even closer to perfect resonance. Were the basin to be enlarged or deepened, whether by natural sedimentary processes or by human engineering, the tidal range might increase further still. Such observations have informed debates about proposals to harness the tides for electricity, since any large structure placed across the bay could in principle alter its resonant behaviour in ways that are difficult to predict. The towering tides shape the entire character of the region. They expose vast mudflats at low water, which provide rich feeding grounds for migratory shorebirds that arrive in their millions. The same tides scour the soft sandstone cliffs along the shore, carving the dramatic rock formations for which the area is celebrated. In certain rivers that empty into the bay, the advancing tide can form a tidal bore, a single wave that travels upstream against the current of the river. For the communities living along its shores, the rhythm of these immense tides governs fishing, navigation and daily life. The Bay of Fundy thus stands as a striking demonstration of how the interplay of coastal form and the physics of oscillating water can produce a natural spectacle on a scale found nowhere else.