MATERIAL MAKEUP OF THE BAY
Duane K. McCullough
Southern Florida overview image
During the last Ice Age about ten-thousand years ago, much all of Florida Bay was a freshwater realm because global sea levels at that time were lower therefore making this area above sea level. When the ice melted in the northern hemisphere, the sea level rose in this portion of Florida and covered the area with very shallow seawater like it is today.
Geologist believe that about 125,000 years ago, this area was under about 35 feet of seawater. The living coral reef at that time -- which today is about five to eight miles offshore of Key Largo, was Key Largo. The island of Key Largo is made up of billions of petrified coral animal bones as calcium carbonate rock salt with about five thousand years of decay tree and plant material as dirt on top of the bedrock stone.
The foundation of most of the islands of Florida Bay do not have a solid stone base like Key Largo. If one could see all the islands without the trees, they would see berms or small hills of crush shells, mud and marl that have been pushed together by storm waves overtime.
There are no significant beaches in this area of Florida Bay because of two reasons. One is that the water level is so shallow here, there is not enough depth to create big waves -- therefore there are no big waves to crush the shells and shove them up along the shoreline as beaches. The other reason why no large beaches can be found in this area is because the Red Mangrove tree seeds and roots will create a mangrove shoreline before a beach has a chance to form.
If we look at the overview map of southern Florida again we will see that most of the area known as the Everglades is a vast slow moving shallow freshwater river full of saw grass and dotted with hammock islands. This seasonal river with glades that seemed to go forever, begins near Lake Okeechobee and flows southward through southern Florida and into the wetland estuary known as Shark River which drains into the area just north of Cape Sable known as Ponce de Leon Bay.
Everglades National Park is roughly the last third of this vast realm with the first third south of Lake Okeechobee now being used by agricultural interest -- mainly sugar farming companies. Much of the eastern side of the middle third area of the Everglades realm is being used for water reservoir reasons to supply the growing metropolitan cities of the southeast coast of Florida like Ft. Lauderdale and Miami.
When the prevailing southeastern Summer breezes reach the mainland of southern Florida they steam up over the warmth of the land in the morning and melt as large thunderstorms in the afternoon. Viewing from Key Largo, it is a common sight to see in the Summer a fifty to sixty thousand foot mountain of an afternoon thunderstorm growling over the mainland of southern Florida.
In fact, the anvil of the daily afternoon Summer thunderstorm event will catch the golden rays of the setting sun and, together with some lightning action, can give a spectacular entertaining sky show to end the days activities in style. Many late afternoons, phenomenal light events called "Sun Dogs" can be also seen on either side of the sun projecting small rainbow-like colors from the high ice crystal cirrus clouds that paint the sky.
Wind speed on the bay can be measured by the wave conditions on the water -- for example, white-caps begin forming at about 12 knots. At 15 knots there is no question that white-caps exist -- while at 17 knots, small foam lines which are caused by repetitive wave breaking activity begin to form and run parallel with the wind. And when the wind is blowing 20 knots, there is no question that foam lines exist because they are very distinct.
Early in the morning at sunrise during Summer, a squall line of thunderstorms can come blowing in from the Atlantic Ocean and bring a good rain to the bay area. If the trade winds slack off a little, it is common to see several waterspouts draining from the long line of clouds hovering over Key Largo as the heat of the Sun boils the humidity into more clouds over the island.
A large waterspout can truly mesmerize the mind with its ability to form a huge column of spinning water over a thousand feet high. The geometry involved in how the water moves along the column is amazing -- sometimes there exist an inside core of spinning water that moves opposite to the outside sheath of spinning water. And if a waterspout hits land, the base area of the column can breakup and the action may take several seconds to reverberate up the column until it spins apart.
Watching the wispy movement of falling blue-gray rain during the approach of a large thunderstorm over the bay as it creates a thin line of blowing spray upon the aquamarine emerald waters of the bay is a very entertaining experience. However, when everything begins to turn dark gray and Mr. Sparky visits nearby with a loud crack, it is time to boat away from the storm to friendlier skies.
Thunderstorm movement can sometimes trap a boater in an area of the bay and relative protection can be found within a nearby mangrove creek under the trees. But during the rainy season, many creeks are full of hungry mosquitoes -- so be sure to carry insect repellant onboard if you visit in your own boat.
Two feet of rain can fall out of the sky in just two days during the rainy season -- which suggest that many nearby shallow bodies of water like Buttonwood Sound, Blackwater Sound, Tarpon Basin and Lake Surprise located in the upper Florida Bay area could have once held fresh rainwater much of the year. A Sound is a body of water that contains saltwater while a Lake is a body of freshwater. A Basin or a Lagoon is a body of water that could be salt or fresh depending on whether it is the dry or wet season.
Some studies have suggested that nearly 80% of the freshwater in Florida Bay comes from direct rainfall -- which leaves roughly 20% from the mainland. However, there is a new understanding about of an ancient freshwater aquifer formation under the bedrock areas of the bay that is linked to freshwater areas higher on the mainland.
This "river of sand" or "gravel zone" area is called the Long Key Formation, and is found at depths that may average 100 feet near the middle of the bay. Aquifers provide aquatic pathways to surface sites areas that would be considered as springs. Several large bayside holes in the bedrock near Key Largo are believed to have a path under the island to the Oceanside because water current is seen moving out of them during strong easterly wind events.
The hydrological makeup of upper Florida Bay began to change about a century ago when the construction of the Overseas Railroad roadbed between the Florida mainland and Key Largo began to block the natural water flow between the bay area and tidal waters from other large lagoons northeast of the bay near the lower end of Biscayne Bay.
But what significantly effected the natural water flow in upper Florida Bay was the construction of the 8 foot deep by 80 feet wide channel of the Inter-coastal Waterway that was dredged through the area just prior to World War 2. The Inter-coastal Waterway channel allowed saltwater from the middle of the bay and points north to flow into what was once fresh or brackish water containment lagoons that held rainwater in the dry season.
And yet another significant change has taken place to the hydrological makeup of upper Florida Bay over the years. First built during World War 2, a pipeline from a shallow well on the mainland located near Florida City north of the area has been diverting fresh drinking water to the Florida Keys from the Everglades. In the early eighties, the government built a newer pipeline to replace the old one and ever since then about 15 million gallons of freshwater is being pumped to the Florida Keys every 24 hours, 7 days a week.
That's a lot of freshwater not naturally flowing into upper Florida Bay from the mainland area overtime. Later in the tour, other man-made canal construction projects will be mentioned that have also altered what was once a freshwater estuary with natural springs into a saltwater region with saltwater animals.
Major hurricanes can also effect the hydrological makeup of upper Florida Bay from what it is to something different. Storm surge activity during a major hurricane can create new passes between the lagoon barrier islands that can allow more or less salty seawater to enter the area.
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