For hundreds of years, English speakers have called the highest tides in each monthly lunar cycle spring tides. Spring tides “spring forth,” and coincide with full moons and new moons when the sun and moon are aligned. The lowest tides in the monthly cycle occur seven days after the spring tides, at times when the sun and moon pull at right angles to each other. Anglo-Saxons called these neap tides, from a word meaning “without force.”
Last October and November brought unusually high tides that flooded areas from Charleston to Miami Beach. Those tidal surges were the highest ever recorded except during hurricanes. The water level was more than a foot above the predicted highs, and flooded the roadway that links Savannah to Tybee Island. Anyone who tuned into the Weather Channel probably began hearing about “king tides,” which is a term now taken to mean the highest spring tides of the year. It seems odd that the highest “spring” tides occur during the fall, but that is true in Apalachicola Bay as well as along the Atlantic Coast.
The water level in Apalachicola Bay is normally highest during summer and fall, and lowest during the winter. For example, the predicted highest tide in January was 1.4 feet above mean low water, but it is forecast to be 1.87 feet next October. This difference seems odd because the water level in the Gulf of Mexico does not change seasonally. The main reasons for seasonal fluctuations include reduced water flow down the rivers and changes in the distance from the earth to the moon. Admittedly, this year has been different due to rains and flooding along the river causing abnormally high tides in the bay.
Anyone who makes a living on Apalachicola Bay or the nearshore Gulf tends to check the tides before leaving the dock for the simple reason that every fish, bird and crustacean is attuned to the tidal cycle. High tides allow baitfish to move deep into the cord grass, and wading birds and predatory fish follow the bait. Low tides concentrate fish in the deeper channels around the oyster bars, and fishermen again “follow the bait.”
In contrast, landlubbers may pay little attention to tidal cycles until they come to Franklin County and want to book a fishing trip. Tuning into the moon and tides is part of the charm of St. George Island. However, tides govern more than when fish, birds and sea turtles are active; they also govern where we can build homes. That is because any home along the beach or waterfront has to be located well above the mean higher high water mark. This line of thought leads us to another Anglo-Saxon term, namely, a “sticky wicket“: the mean higher high water mark is changing. It is getting higher as sea levels rise, and has been doing so ever since Apalachicola was founded.
Although the state government in Florida does monitor high water marks in order to define a Coastal Construction Control Line, the task of tracking our changing tide levels goes beyond state capabilities. To survey the entire coastline of the United States is a big job, and it falls to the federal government, specifically the National Oceanic and Atmospheric Administration, or NOAA. The way this bears on the topic of king tides is that NOAA maintains an automated data collection station in the mouth of the Apalachicola River and at many other sites along the coast, as well as buoys offshore. Maintenance of these stations falls to the Coast Guard.
The local tide stations collect information about current water levels, wind speed and direction, river flow, and atmospheric pressure. All the information is available to you in real time by looking up Apalachicola at the NOAA Tides and Currents web page. The way this relates to king tides and spring tides is that the present measurements for tides are compared to values predicted from a complicated equation that takes into account the pull of the moon and sun, local geography, and factors that are far beyond my ken.
Fortunately, you do not have to understand the math to see what is going on, because NOAA shows a chart that contains measured and predicted values (See illustration). What immediately stands out is that the measured value is often higher than the predicted value. At this moment, the measured water level is nine inches higher than predicted. Why would that be?
Several factors affect the measured and predicted values. The predictive model is based on an average mean lower low water level that has been measured for five years in Apalachicola and the Gulf of Mexico, or 19 years along the Atlantic and Pacific coasts. When the tidal stations were set up, several benchmarks were chosen as points of reference so each tide station fits into a survey grid. Over time, deviations from those bench marks can be measured very accurately, and water levels have risen since the stations were set up.
Besides the changes in average water levels, high tides are strongly affected by recent rainfall, river flow, wind speed and direction, and atmospheric pressure. The abnormally high king tides on the East Coast last October happened because the strong gravitational pull from a supermoon combined with onshore winds, a slower Gulf Stream current, and rising sea levels. The factors affecting actual tide levels in Apalachicola Bay are a bit different, but the overall picture is the same: tides can be abnormally high, and the worst case scenario would combine high tides during a fall supermoon with rains, onshore winds, or, worst of all, another hurricane.
King tides are a worldwide phenomenon, in fact, the term was first introduced by Australians who noticed exceptionally high tides “down under.” At this point, scientists do not officially accept the term, king tide, preferring to call them “perigean spring tides.” The reason king tides occur is that the moon is closest to the earth at predictable times (the perigee, when supermoons occur), and you can literally look the dates up in astronomical tables. This year, the lunar cycle brings the moon closest to the Earth first during the new moons of April 7 and May 8, and again during the full moons of Oct. 16 and Nov. 14.
If you are a bay watcher like me, keep your eyes on the tideline during these weeks, and you will start to get a grasp on how king tides are affecting our area. The web page that provides tide predictions and water levels for the Apalachicola River mouth is tidesandcurrents.noaa.gov
James Hargrove retired from the University of Georgia in 2012 and now resides on St. George Island. An amateur naturalist and historian, he volunteers as a docent at the Cape St. George Lighthouse Museum, and is working on a pictorial history of the lighthouse with leaders of the St. George Lighthouse Association.