It is surprising how many people do not know the difference between high and low tide, not to mention ebb and flow. It is a commonly held belief that the word "ebb" is synonymous with low tide and "flood" is synonymous with high tide. Not so.
Boaters cannot learn the best time to take a boat across the bar at the mouth of an estuary like the Columbia River or the Golden Gate without first leaning the vocabulary. Avoiding the worst conditions requires timing your crossing to suit the tides and currents, and that means knowing the difference between a tide and a tidal current.
EBB AND FLOOD
It's not the same as high and low tide. They are instead directional adjectives, which indicate the direction of the tidal movement.
"The tide was at ebb" means that the current was going out. Flood, on the other hand, means that the current was going in, and maximum flood is the time when the incoming current reaches its maximum velocity. This occurs about half way between the time of low tide and the time of the next high tide. Maximum flood is not, as many people assume, a condition that occurs when all the tide is in. That is what is known as high tide.
At the time of high tide, or shortly thereafter, the current becomes slack, which means it is neither ebbing nor flooding but just standing still. This happens about 30 minutes to an hour after high tide.
The tide then turns and begins to ebb until it reaches low tide, about six and a quarter hours after the time of high tide.
Unfortunately, the moon and sun exert their forces on tides, complicating time calculations.The moon's slower transit usually means tomorrow's tides will be about an hour later than today's. The lesser pull of the sun retards or advances the times of the tides slightly. The combination of these forces gives us the varying heights of the tides.
Once we understand all the terminology, it should be simple enough to look up the time of high or low tide in a tide table, and the times of maximum flood and maximum ebb in a tidal current table.
Of course, you probably know that the high and lows arrive at different places at different times.
Added to that is the fact that the maximum depth of the tide could be quite a bit higher or lower, depending on the area.
To calculate these times, heights and current velocities for various locations around require using the tidal offset tables. The Coast and Geodetic Survey, a government agency that preceded NOAA, collected and compiled all this data and created tidal offset tables.
In the old days, we had to calculate it ourselves. But now, we can go on the Internet and find it already calculated for us at NOAA's tide and currents Web site.
MORNING FLOODS HAVE LESS WINDS
When we have the data at our fingertips, we can plan a bar crossing for a time when both the currents and the winds will be most favorable. There is not much wind before 11 a.m., but there is a lot of wind in the afternoon. All of your bar crossings should be planned for the times around slack water. At the mouth of an estuary, the flood currents are slightly slower than the ebb currents. So plan to cross in the last half hour of the morning flood, and if do it before high slack, which occurs between 2 a.m. and 2 p.m.
On the Pacific coast, because of the prevailing westerly winds, do take precautions to avoid crossing during an afternoon ebb. When the tide is running out in the afternoon, the sea breeze will be blowing in and can create some steep seas.
