Tides

Gravity

When we think of gravity, we think of what keeps our feet on the ground and why things fall to earth. Earth’s gravity is the most significant pull that we experience from a large mass. The moon’s pull comes in a distant second, while the pull of the sun comes in at approximately half that of the moon. Despite having a much larger mass than the moon, the sun is much further away from planet earth than the moon. The force between two bodies is proportional to their combined mass and inversely proportional to the square of the distance between them.

Although we are not overly aware of the gravitational pull of the moon or the sun, the earth’s oceans are impacted, primarily because they are most fluid part of the planet.

The gravitational pull of the moon actually causes the water in the earth’s oceans to bulge out toward the moon, while the spin of the earth produces a bulge on the opposite side. This pull on the earth’s water combined with the spin of the earth results in a rise and fall of water levels all over the planet.

“Moon Tides” occur when the sun, the moon and earth are in alignment, which occurs every 14 days. The moon is either new or full. The closest part of the earth to the moon will experience the highest tide. “Moon Tides” are also known as “Spring Tides” and they have nothing do with the season of spring.

“Neap Tides” occur when the sun and the moon are at right angles to each other, which occurs every seven days. The moon is either in the first quarter or the last quarter.

The point at which the moon is nearest to the Earth in it’s monthly orbit is called it’s Perigee.

The point at which the moon is farthest from the Earth in it’s monthly orbit is called it’s Apogee.

There 4 types of tides;

      • Semi-diurnal Tides (two high tides and two low tides in a lunar day)

      • Diurnal Tides (one high tide and one low tides in a lunar day)

      • Mixed Semidiurnal Tides (two high tides and two low tides in a lunar day, each of a different size)

A lunar day equals 24 hours and 50 minutes.

As a child growing up in Newfoundland, the saying was; “6 hours to rise, 6 hours to fall, a half hour later each day”. 

Most coastal areas experience Semi-Diurnal Tides, including the eastern seaboard of North America.

Tidal Currents

The tidal range in the open ocean is about 2 feet. The tidal range near the coasts varies significantly depending the the coastal configuration.

A rising tide interacts with coastal areas by flowing into bays, inlets and river estuaries. This current is referred to as the “Flood Current“.

Conversely, with the falling tide, the tidal current is reversed as the inlets and river estuaries drain back into the ocean. This current is referred to as the “Ebb Current“.

“Slack Water” or “Slack Current” occurs when the tidal current shifts from Flood to Ebb and vice versa.

On a rising tide, large volumes of water are pushed into the bays, inlets and river estuarys along the coast. These inlets usually get narrower as they go inland. They also get shallower. This causes the water to speed up and bunch up, which increases the tidal range.

The above screen snip from windy.com is a snap shot of the tidal currents on Canada’s east coast during a rising tide. The dark blue indicates areas of little or no tidal current, while the green represents tidal currents of less than 0.5 kts.

Areas of increased current include;

        • the Strait of Belle Isle

        • the St. Lawrence River estuary

        • the Bay of Fundy

The flood current in the Strait of Belle is about 1.5 kts. in southwesterly direction, while the flood current in the Cabot Strait is about 0.5 kts. in northwesterly direction.

The screen snip is showing significant “Flood Current” in the Bay of Fundy. The highest tides in the world are recorded in the Bay of Fundy. The tidal range in the inner bay approachs 50 feet.

The St. Lawrence River Estuary

What is most interesting in this snap shot is that at the same time that the Bay of Fundy is experiencing a “Flood Current”, the St. Lawrence River estuary is experiencing an “Ebb Current”. In other words, the estuary is still draining from the last high tide.

So, why did that happen?

At some point after the start of the rising tide, the water near the mouth of the river (Pointe-des-Monts) begins to flow upstream.  A large tide will reverse the river flow as far upstream as Portneuf, which is 250 nautical miles upstream of Pointe-des-Monts.

This is a good example of where the incoming tides are funneled into a shallow, narrowing river, resulting in an amplified tidal range upstream. The tidal range peaks as it passes Isles-aux-Coudres, where the range is 23 feet. The tidal range at Quebec City is about 15 feet.

It should also be noted that as the tidal flow moves upstream, the time of high and low tide is lagged.

Depending on the width and depth of the channel, there are some places where the “Flood Current” may occasionally exceed 4 kts. Because the “Ebb Current” includes the river flow, it will often exceed 4 kts.

This Atlas of Tidal Currents is published by Fisheries and Oceans Canada and is available at: https://publications.gc.ca/collections/collection_2022/mpo-dfo/Fs72-51-2-2022.pdf

A Sample Page from the Atlas

The graphics in the above sample page will help you take advantage of the currents.

Based on my experiences cruising the lower St. Lawrence River, I would highly reccommend that you consult local knowledge along the way. The boaters who live in the area know exactly when you should depart to catch the “lunar express”.

Departing at the optimun time, means that you will add as much as 6 kts. to your speed through the water. Depart at the wrong time and you may find yourself subtracting as much as 6 kts. from your speed through the water.