Tides


Those of you who have spent some time on the beach have probably noticed that the water level of the ocean does not stay the same all day long. Although waves continually wash the shore, the actual water level changes as the day passes. It rises for a period of hours until it reaches its highest level, and then it begins to fall, or recede, to its lowest level. This rise and fall of the ocean is known as the tide. Its highest and lowest levels are, of course called high and low tides. The incoming water is called the flood current and the outgoing water the ebb current.

Many years ago people thought of some strange reasons for these changes in water levels. Some believed the earth was a living animal and the tides a result of its breathing. Others, who thought the ocean’s waters were the earth’s blood, decided the tides were its pulse, caused by the beating of its great heart. Early followers of the religious prophet Muhammad believed that the rise and fall of the waters were caused when the “Angel of the Sea” placed his foot in and out of the ocean. Primitive natives thought the tides were a sign of a sea god’s anger and made human sacrifices to please him.

Phases of the moon showing spring tides and neap tides

As scientists learned more about the earth, they began to realize that the pattern of the tides seemed to follow the lighted shapes (phases) of the moon. But, until Sir Isaac Newton, an English scientist, published his theories on universal gravitation in 1687, it was not known just how the moon and tides were related.

A simple way to picture how tides occur is to imagine the early completely covered with a layer of water. As the moon circles the earth, its gravity pulls the water out into a bulge on the side of our planet nearest it. The force of the spinning earth (centrifugal force) causes the water to bulge on the opposite side of the earth. These bulges make the high tides. Half-way around the earth on each side between these two bulges occur the low tides. As the earth turns, the water level rises or falls.

If the earth were covered with water of the same depth, there would be two identical high and low tide cycles over the entire surface during a complete turn. However the shape of the land masses and the size, shape, and depth of the ocean basins, bays, gulfs, and estuaries interfere with this perfect cycle. As a result, some coastal shorelines have two high and low tide cycles daily, others only one, and still others a mixture.

Because of the shape of the basin of the Gulf of Mexico, the Texas Coast receives mixed tides. This means that some days we have two high and low cycles, others only one, others one high and two lows, and still others two highs and one low. Coastal newspapers publish tide charts to tell what times these high and low tides will occur.

The range of the tide, which is the difference in the water level between high and low tide, is also affected by the physical features of the land and ocean. Some coastal shorelines have a range of less than one foot, but the Bay of Fundy in Novia Scotia has a tremendous range of fifty feet or more.

The range along the Texas Coast varies from a few inches to a couple of feet. This may not sound like much of a rise, but you must remember that it is the actual water depth. This means that on a fairly flat beach, the water can move onto the shore twenty feet or more. Keep this in mind if you camp on the beach, or you may wake up in the middle of the night and find the incoming tide has moved into your tent while you slept.

High Tide: As the tide washes in, the incoming current stirs up nutrients and brings oxygen-rich water to stagnant pools. Ocean fish also move in to feed in the shallow waters.
Low Tide: As the tide goes out, it carries with it dead or decaying vegetation as well as many marine animals. Those creatures left lying on the beach are quickly eaten by the birds.

Tides also are pulled by the sun’s gravity. Twice each month, when the sun and moon pull together on one side of the earth (new moon) or pull on opposite sides (full moon), the combined gravity causes the highest high tides, called spring tides. The name spring tide has nothing to do with the season of the year, but comes from the Anglo-Saxon word springan, which means “to rise.” Between the two spring tides, during the moon’s first and third quarters, the lowest high tides, called neap tides, occur. At those times, the pull of the sun is at a right angle to the pull of the moon, and the two forces tend to cancel each other.

You would think that the pull of the moon and sun and the physical features of the land and water would be enough things affecting water levels, but wind and weather also enter the picture on occasions, although they are not true tidal actions. A strong wind blowing from the ocean to the shore can create a landward water current that raises the water level and causes higher tides while it is blowing. A strong wind blowing from the shore to the ocean will cause the opposite to happen. When the barometric pressure rises one inch, the additional weight of the atmosphere can lower the water level about thirteen inches. A drop in pressure raises the water level.

The combination of a hurricane-type wind and the low barometric pressure associated with such storms can produce gigantic waves that are commonly, but mistakenly, called tidal waves. They really have nothing to do with regular tidal action and would not happen under normal conditions. Other misnamed tidal waves are caused by underwater earthquakes, volcanic activity, or landslides. To correct the idea that these destructive waves are caused by the tides, the Japanese name for earthquake-type waves, tsunami (tsso-NAH-mee), is now being used.

Inland waters also feel the gravitational pull of the moon, but their tides are so small as to be unnoticed. The tide rise and fall in Lake Superior is only about two inches.

People who live along the coast are probably more affected by the rise and fall of the water, but tides are important to everyone. Their daily flushing action helps keep harbors and bays clean by picking up waste material and carrying it out to sea where it can settle to the bottom. They also help to wash out silt that may have settled at the mouth of a bay.

A world of marine creatures exists in the “Kingdom of the Tides,” the area rocked by the incoming and outgoing waters, and tides play an important part in the ecology of the bay and estuary areas. As the tide sweeps in, it covers the salt marshes, stirs up nutrients (food), and brings rich oxygen to stagnant pools that remain from the previous high tide. When the incoming tide brings its food over the oyster reefs, the oysters open their shells to feed. If you were to take an oyster from the Atlantic Coast to a laboratory in El Paso, for a period of time it would continue to open and close its shell at the times the tide washes in and out on the Atlantic Coast. However, after a while, the animal would respond to the gravitational force of the moon in El Paso and begin to open and close its shell to feed as if El Paso were on the ocean.

Ocean fish ride in on the incoming tides to feed in the flooded shallows. For this reason, better fishing days are usually those which have extra strong incoming tidal currents to bring food and fish into the bays. Flounder fishermen wading in the shallows during an incoming tide may have better luck gigging this tasty fish since low tides expose the shallow feeding areas and force the flounder to move out into deeper waters.

As the tide recedes, many marine animals bury themselves in the wet mud to await the next tide. Birds swoop down to feed on the ones left lying on the beach when the tide goes out.


Additional Information:

Ilo Hiller
1983 Tides. Young Naturalist. The Louise Lindsey Merrick Texas Environment Series, No. 6, pp. 130-133. Texas A&M University Press, College Station.

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