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Tidal Diagrams courtesy of Fundy Discovery Site
Our Tides
Text generously provided by Dr. Roy Bishop
What causes tides?
Gravitational influence of the Moon
Gravity becomes stronger with decreasing distance. Thus on the side of Earth nearest the Moon a tidal bulge occurs. Earth is attracted toward the Moon too, leaving a second tidal bulge behind, on its far side. As Earth rotates once per day beneath those two bulges, most regions on Earth experience two high and low tides. The Moon orbits slowly eastward in the same direction that Earth rotates, causing the tide cycle to be longer than 12 hours by the extra 25 minutes required for Earth’s rotation to catch up to the Moon.
Why does the tide range vary?
Phase of the Moon
The gravitational influence of the Sun also affects Earth’s oceans, but because the Sun is much further away, its two tidal bulges are smaller than those raised by the Moon. As the Moon goes through its monthly cycle of phases, the Sun-Earth-Moon angle changes. At new and full moons, the lunar and solar tides are aligned resulting in larger tides, called spring tides. A week later at the quarter moons when the Sun and Moon are at right angles to Earth, the smaller solar tide no longer adds to the lunar tide resulting in smaller tides, called neap tides.
2. Closeness of the Moon
The Moon’s orbit around Earth is not circular. It is an ellipse (oval shape), thus the Moon’s distance from Earth varies. When closest to Earth (perigee) the Moon’s tidal influence is greater. When furthest from Earth (apogee) the tides are smaller. At any place on the oceans, the largest tides, known as perigean-spring tides, occur when the full, or new, moon (spring tides) coincides with when the Moon is closest to Earth (perigee). Those enhanced tides recur about every seven months. Fundy tides are unique in that they respond more to the perigee-apogee influence than they do to the spring-neap influence.
Why are Fundy tides so large?
Resonance—like a push on a swing
Imagine a parent pushing their child on a swing. A gentle, repetitive push by the parent will sustain a large back-and forth motion of the swing. The parent must time the pushes to closely match the natural period of the swing, a condition called resonance. Fundy tides are driven not directly by the Moon and Sun, but indirectly by the smaller tides in the Atlantic Ocean. When a rising Atlantic Ocean tide crosses the edge of the continental shelf into the shallow Gulf of Maine, it travels as a long wave to the head of the Bay of Fundy, reflects or bounces back, and returns to the edge of the continental shelf, its speed determined by the depth of the water. When the wave reaches the edge of the shelf, the dramatic increase in ocean depth causes most of the wave to reflect once again and travel back toward the head of the Bay of Fundy, much like a swing. The time for this cycle is close to the 12.4-hour period of the lunar tide, which means that each Atlantic tide gives the previously reflected wave an almost perfectly timed push. It is that small, repetitive, well timed push that drives Fundy’s tides.
What is a Tidal Bore?
A tidal bore is a tumbling wavefront which moves upstream in a river, announcing the arrival of an incoming tide.
Tidal bores occur in regions of the world that have large tides. However, a tidal bore does not occur in every river flooded by a high tide.
Three conditions are necessary to form a tidal bore:
Almost flat riverbed – The riverbed must have a very gentle, downstream slope.
Higher riverbed – The riverbed must be well above the level of the low tide so that the tide is rising rapidly when it enters the part of the river where a tidal bore might appear.
Shallow river water – The water in the river prior to the arrival of the bore must be relatively shallow. If a tidal bore encounters water that is much deeper than the height of the bore, the bore collapses into a barely noticeable, smooth surge of rising water.
Why does the size of the tidal bore in the Salmon River vary?
The largest tidal bores in the Salmon River occur on the days of perigean-spring tides (the Moon closest to Earth and at its new or full phase). Occasionally, because of much rain and/or rapid snow melt, the river may be too deep for a tidal bore to form. This is most likely to happen if the tide is a minimal apogean-neap tide (the Moon furthest from Earth and at its first- or last-quarter phase).
for more information on Tides and Tidal Bores, visit:
Fundy Discovery Site (Lower Truro) Tides & Tidal Bores – Fundy Discovery Site – Be Moved By The World’s Highest Tides – Truro, Nova Scotia
Fundy Ocean Research Centre for Energy (Parrsboro) Visit | FORCE (fundyforce.ca)
