The level of Lake Okeechobee, which peaked at 17.2 feet earlier this month, has started to slowly go down.
The lake level was around 13.5 ft. the week before Hurricane Irma hit, and around 13.7 ft. the Friday before the storm.
In about 30 days, rapid inflow of heavy rainfall from the north pushed the lake level up 3.5 feet, to 17.2 ft. The lake’s regular operating schedule ranges from a low of 12.5 ft. to a high of 15.5 ft. Levels above 16 ft. are harmful to the lake’s marshes and ecology, according to Florida Audubon. Levels above 16 ft. also increase the risk of breach of the Herbert Hoover Dike, the 143-mile earthen berm that surrounds Lake Okeechobee.
One inch of water on the lake is about 12 billion gallons; so an increase of 3.5 feet (42 inches) is 504 billion gallons.
Due to the rapid inflow from the north since Irma, the U.S. Army Corps of Engineers has released as much water as feasible east to the St. Lucie River via the C-44 canal through the lock at Port Mayaca, and west to the Caloosahatchee River through the Moore Haven lock.
This year has been an unusual one for water flow at Port Mayaca, with water flowing from the C-44 into the lake right until the hurricane hit, and water flowing the other direction afterward. The flow at Port Mayaca is gravity-fed, so if the water level is higher in the canal than the lake, the flow goes west, and if the level of the lake is higher than the level in the canal, the flow goes east.
So far this year, a net total of about 21 billion gallons of excess freshwater has entered the St. Lucie River from Lake Okeechobee. For the net total flow, subtract what went into the lake at that structure from what went out.
According to SFWMD data, from Sept. 10 to Oct. 15, 49 billion gallons flowed from the lake through the lock at Port Mayaca to the C-44 canal. During an unusual rain event during that period, 1 billion gallons flowed from the C-44 through the lock into the lake. So the total from the lake to the C-44 for that period was 48 billion gallons.
Prior to Hurricane Irma, water was flowing the other direction. From the start of the rainy season, about 27 billion gallons flowed from the C-44 into Lake Okeechobee.
Subtracting the 27 billion from the 48 billion, leaves 21 billion gallons of total excess freshwater entering the St. Lucie watershed from Lake Okeechobee.
That’s still a lot of water.
But when it comes to phosphorus load, the two sides of the equation nearly even out.
Per SFMWD data, the average phosphorus load in basin runoff in the C-44 is about 300 parts per billion. Water in Lake Okeechobee averages just over 100 ppb. In July, the average was around 128 ppb, per SFWMD data. The turbidity caused by the rapid flow of water entering the lake from the storm likely raised the phosphorus levels in the lake. But even so, the water from the lake contains less than half the concentration of phosphorus as the basin runoff.
Of course, biologists will point out that the primary problem with excess freshwater entering the St. Lucie from the lake has to do with lowering the salinity levels. Even if it is as clean as rainwater, the excess freshwater is detrimental to the estuaries.
And even at 100 ppb, the phosphorus level in the lake is too high. The FDEP target for the lake is 40 ppb. However, with the exception of direct rainfall, all of the water entering the lake is many times the target level. Per FDEP officials, unless the water is cleaned before it goes into the lake, there is little hope of bringing down the phosphorus levels in the lake.
The current drainage system moves water much faster than the natural system, and managing that manmade system requires considering the entire flow, and all of the sources of phosphorus entering the lake and the estuaries.