When I was 12-years-old, I spent part of the summer with my father in Oregon. One of the most memorable moments was during a weekend at the shore. Early one Saturday morning I went out to play with several other children on a broad, dry mud flat. By mid-morning my father and I were out fishing, rowing in several feet of water where just a few hours earlier I had been running on dry land. That was this mid-western kid's introduction to the concept of tides.

People who live in coastal communities are familiar with the clock-like rise and fall of the waters. It's regular and predictable. The times of high and low water advance about 50 minutes each day, rising and falling in a rhythmic pattern. The basic mechanics are simple, but variations in the shape of different coasts make the results very different.

THE SUN AND MOON

The vertical movement of water we call the tide is created by the gravitational pull of the moon and the sun. While the moon is much smaller than the sun, it is a great deal closer to the Earth and thus becomes a controlling force. Typically, a given area will experience high tide when the moon is on a certain bearing.

Both the sun and the moon affect the rising of the waters. When the sun and moon are lined up (during a new moon and full moon), their effect is cumulative, causing water to rise higher and fall lower than average. These are called spring tides and they have nothing to do with the season, as evidenced by the fact they occur every two weeks. When the sun and moon are at right angles (sometimes called "at the quarters") their total effect is less and the tidal range (the difference between high and low) is less. High tide is not as high and low tide is not as low. These are called neap tides.

The distance between the moon and the earth also varies. When the moon is closest to the earth it is said to be in perigee and tidal ranges are greater. When the moon is in its farthest position it is in apogee, and tidal ranges are less.

If the perigee coincides with a new or full moon, high tides will be very high. Add an onshore storm to that and you have a recipe for coastal flooding.

HIGH AND LOW TIDES

Most, but not all, places experience two high tides, and two low tides, every day. It is reasonable that the tide is high when the moon is in sight. But what about the other tide? Gravitational attraction produces a "lump" of water that goes around the world under the moon. On the opposite side of the earth is another lump created by the centrifugal force of the earth and moon rotating around their common center of mass (about 1,100 miles below the earth's surface).

Usually the two daily tides are not exactly the same height. In some cases the difference is quite noticeable. Chart depths will be noted as either "Mean Low Water" or "Mean Lower Low Water." "Mean," in this case, is a mathematical term that means "average." During an extreme spring tide, the water may be lower than charted, so you need to pay attention to the tide tables.

Tide tables are commonly available just about everywhere. Simple ones are often given away in bait shops. Most chandleries sell tide tables in book form. And many navigational software packages have tide information. These tables show a more complete set of tidal predictions. They give detailed predictions for a limited number of reference stations and offsets (both for time and height) for a great number of subordinate stations. The depths indicated are relative to the chart datum (usually mean lower low water), thus a negative depth (such as -0.3 feet) tells you that at low tide there will be less water than what is shown on the chart.

JAN VERTEFEUILLEGrand Manan Island, at the mouth of the Bay of Fundy, experiences tidal variations as great as 50 feet during each cycle. Shown is a high and low tide view of Whale Cove on the island's north end. JAN VERTEFEUILLE

Most tide books provide a table to calculate the depths for any time between those tabulated, but a good rule of thumb is that the tide will rise or fall one-twelfth in the first hour, two-twelfths in the second hour, three-twelfths in the third and fourth hours, two-twelfths in the fifth hour, and one-twelfth in the last hour of the cycle.

COAST LINE

Tide times and ranges will vary greatly from place to place. This is due to the way the shape of the land affects the incoming water. For example, the North Carolina sounds, which have a huge area of water fed by just a few small inlets, have a tidal range of just a couple of inches, while further north, at the Bay of Fundy, a unique and complicated set of conditions produces tidal ranges of more than 40 feet.

Remember that tide predictions are based on the regular movements of the sun and moon. The actual tidal variation may vary from the predictions due to strong and continued winds, extremes of barometric pressure or heavy runoff from rivers. In most cases the tide tables will be accurate, but be aware of other factors that can modify the predictions.

TIDAL CURRENTS

I don't recall if we caught many fish that day in Oregon, but I do recall looking up and noticing that the shore seemed to be zipping by. The water was running out of the bay and we were being swept out to sea along with it. As the tide comes in and goes out it creates currents. A rising tide, pushing up bays and coastal rivers, creates a current that flows upstream. It is called the flood. The tidal portion of rivers actually reverses their flow four times a day. As the tide goes down again, there is a current flowing toward the sea called the ebb. In a river that has a current of its own, the ebb is often stronger than the flood.

Like the tide itself, these currents are affected by local topography. It would be a mistake to try to guess the time and strength of a tidal current by the state or height of the tide. Floods and ebbs don't necessarily coincide with rising and dropping tides. For example, the presence of a point of land that causes a restriction in Virginia's York River creates a situation in which the currents are three hours out of sync with the tides. You can be riding a flood current a couple of hours after the tide has started to fall.

As the tide moves up a river or a long bay, the area of flooding current moves up with it. Thus, a boat that can maintain the appropriate speed will be able to ride the flood for a long time. For example, if you can maintain a speed of at least 10 knots you can ride the flood all the way up Chesapeake Bay from Cape Henry to Baltimore.

There are tidal current tables just as there are tide tables. They will give predictions for the times of slack water between the floods and ebbs, and give you the speed and direction of maximum currents. They are organized similarly to the tide tables. They usually also include a table for calculating the speed of the current at any time in the cycle. This information is very useful, but it refers only to the currents created by the tides. As with the tide itself, currents can be affected by strong winds, river runoff and other temporary factors, so you need to use your skipper's judgment.

It's been a long time since I was a wide-eyed kid marveling at how the ocean moves and changes. A lifetime of learning and observing later, I'm still fascinated by its silent but irresistible changeling nature.