I regularly get questions about the time and range of the tides in Louisiana coastal marshes. Many of the inquiries are about the time of the published tides from various sources listed as compared to the actual times that the high and low tides occur, and how are they determined.
Ocean tides are mainly caused by the gravitational pull of the sun and moon on the earth. Based on this astronomical data, tides can be predicted for years in advance, but this is not always true in Louisiana. These predictions are great for most of the American coastlines as they have rather stationary boundaries that are directly on or by deep water areas. Yes, I know that some of these other areas have estuaries, but these are usually the exception. Inland lakes not connected to salt water do have tides, but the range is very, very small, usually within less than 1/2 inch.
Louisiana coastline marshes, however, are shallow-water lakes, made up of bayous, bays, canals and lagoons that are part of a living estuary. These waters are easily influenced by weather, especially by a strong or constant wind from one direction. In addition, usually they have numerous bottleneck locations that restrict the water movement, and more important, the water in these areas is usually quite shallow and easily influenced by weather. A large weather high- or low-pressure system over the concerned area can also affect the tides.
And to complicate Louisiana tides, there are two days a month where the impact of the sun and moon cancel each other out. This is called a neap tide. There is no tidal movement caused by astronomical influences, but — and there is always a but — local weather conditions with wind can influence movement.
Not all but most of the tidal data comes from National Oceanic and Atmospheric Administration (NOAA) Department of Commerce. However, published data from the United States, NOAA is the “official” agency for the United States that records, archives and predicts tidal information. Naturally they state a disclaimer that reads: “These data are based upon the latest information available as of the date of your request, and may differ from the published tide tables,” but it is the most reliable source to obtain these tides. Their main web site is located at tidesandcurrents.noaa.gov/tide_predictions.html. However, I find it is difficult to navigate their site and takes some effort to obtain a week’s or month’s tide data for a specific location.
I have found that the tides listed in the Louisiana Sportsman magazine are great and you can view them a least a month at a time.
You can even purchase all of these tides for a year at a time. That makes it easy to plan a trip when there is water movement. They are a great asset to Louisiana coastal fishing.
Advances in technology have helped solve many of the problems associated with the old tidal recording systems. Microprocessor-based technologies allow for customized data collection and have improved measurement accuracy. While older tidal measuring stations used mechanical floats and recorders, a new generation of monitoring stations uses advanced acoustics and electronics. Today’s recorders send an audio signal down a half-inch-wide sounding tube and measure the time it takes for the reflected signal to travel back from the water’s surface. The sounding tube is mounted inside a 6-inch diameter protective well, which is similar to the old stilling well.
The predictions are based on the astronomical data for main recording stations called harmonic stations. Some of the main recording stations in southeast Louisiana are South Pass, Grand Isle East Point, and Calcasieu Pass at the Lighthouse wharf.
According to NOAA, harmonic stations are stations with tidal harmonic constants and tidal datums. Tide predictions for harmonic stations are generated directly from the harmonic constants. Harmonic stations have the greatest capabilities within the NOAA Tide Predictions service for providing predictions with different data intervals and relative to different tidal datums. These capabilities are controlled within the interface and are usually sent to the NOAA main recording site on an hourly basis.
Subordinate stations are historic stations that do not have tidal harmonic constants available. Tide predictions for subordinate stations are generated by first generating high/low tide predictions for a designated harmonic station, called the “reference station;” then time and height adjustments are applied to correct the high/low predictions to the tidal conditions at the subordinate station. Subordinate stations only allow for the generation of high/low tide predictions, and heights will be relative to Mean Lower Low Water (MLLW), the standard chart vertical datum for the U.S. coastline.
These offsets take into consideration the many “choke” points that are between the open sea, the main station and the subordinate location. These bottleneck locations around islands, in bayous, bays and lagoons cause a drag on the flow of the tidal water and delay the water conditions around these subordinate stations.
This “offset” data is an accurate factor, but I, and the harmonic stations do not take into consideration the weather at the locations. A mild prevailing wind for several days or a strong wind for a brief time can negate the total tidal movement in the marsh areas and even drive it backward. This is especially true when the tidal range is less than two feet. These winds can actually reverse the water movement in adjacent passes or in the marshes themselves.
When checking the tides for your next fishing trip, keep a watchful eye on the weather, especially for the winds in your area. Be sure to include data from the offshore buoys that indicate the offshore winds as they indicate how much water is “stacked” along the coastline. In addition, a strong weather high or low over the concerned area can also affect the tides.
Look up the tide data for your area, time and date and use the information from all sources to make an intelligent decision about your trip.