Geoscience Reference
In-Depth Information
ces are at right angles to each other, diminishing their combined effect and producing the smallest tides of the
month, or neap tides. The extremes of tides, both high and low, vary over the course of the year, again as a res-
ult of the relative positions of, and distances between, sun, moon, and Earth.
These astronomical factors influence the range of tides the world over. It follows, then, that the conditions
that produce the extreme tides in the Bay of Fundy have a local origin. The laws of physics tell us that every
basin has a characteristic period of oscillation, which means that once set in motion, waters within it will slosh
back and forth with a regular rhythm. Where an open bay communicates with the sea, however, the influence
of the lunar-dominated tides must also be taken into account. If the basin and ocean tides are rocking back and
forth in harmony with one another, they are said to be in resonance. This is what happens in the Bay of Fundy
and largely accounts for the great tides experienced there.
When applied to water in motion, resonance is more commonly referred to as the “bathtub effect.” A home
experiment can easily demonstrate what happens on a grand scale in the Bay of Fundy. Draw a shallow bath
and give a push to the water at one end of the tub. A wave will travel to the other end and be reflected back.
Just before it reaches your hand, give another push and you will observe that the water begins to slosh back
and forth quite dramatically. In the bay, the moon acts as the hand in the bathtub analogy; it has been shown,
however, that it is not the bay alone but the Bay of Fundy and the Gulf of Maine acting as a single oceano-
graphic system that produces this effect. The period of time it takes for the tidal wave, or bulge, to travel from
the mouth of the bay to the head is only nine hours—not long enough to cause resonance. When the bay and
the gulf are considered together, the resonant period is 13.3 hours, close enough to be in resonance with the
12.42-hour, moon-forced tidal system.
The tidal range increases progressively from the mouth of the bay between Digby, Nova Scotia, and Saint
John, New Brunswick, where it reaches heights of 3.5 meters (11.5 feet), to the head of the bay, where so-
called mega-tides, in the range of 14 to 16 meters (46 to 52 feet), occur. As the tide flows into the bay—a
volume of water equal to all the rivers on the planet combined—it is forced into a shallower and narrowing
space, and, in effect, piles up. This pileup produces in some of the tidal rivers at the head of the bay a hydro-
graphic phenomenon known as a tidal bore. A bore is the leading edge of the advancing tide that forms a tum-
bling wave front, more than a meter (3 feet) high. It often travels far inland before dissipating when it is finally
overwhelmed and dampened by the volume of rising water behind it.
Upwellings of Life
The Bay of Fundy has been aptly described by biologist and bay expert Graham Daborn as “an ecosystem with
a biological pump at both ends.” In the lower end, or outer bay, the tide rips—which gave their native son
Joshua Slocum such a thrashing as they boiled over the reefs around Brier Island— pump nutrients from the
seafloor up into the photic zone. In the upper end, or inner bay—the part of the bay that includes the Minas
Basin and Chignecto Bay—salt marshes and mudflats act as the biological factories, pumping their production
into the marine zone as the tides ebb and flow.
The turbulence that characterizes the waters at the mouth of the Bay is associated with an oceanographic
phenomenon known as upwelling. Upwelling usually occurs near the coast, where prevailing winds move wa-
ters away from shore, and colder, deeper waters move upward to replace them, bringing their bounty of nutri-
ents. This phenomenon is most frequently observed on the west coasts of continental margins, notably off Ore-
gon and Washington, Peru and Chile, and Morocco and southwest Africa. Most upwellings are wind-gener-
ated. In the Bay of Fundy, however, the prevailing winds shift with the seasons, from northwesterly in winter
to southwesterly in summer, suggesting that upwelling is not primarily the result of wind action. In Fundy, as
should be no surprise, it is the bay's great tides themselves that are the generators of upwelling. As the strong
tidal currents pass around the corner of the coastline near Yarmouth, at the southwestern tip of Nova Scotia—
which is oriented at right angles to the direction of the current—they create an upwelling effect in the same
