By Katrina Elsken
LAKE OKEECHOBEE — Lake Okeechobee has dropped below 12 ft. (above sea level), causing an end to the releases to the St. Lucie Canal at Port Mayaca. Currently no water is flowing into the St. Lucie canal at Port Mayaca. The target rate at the St. Lucie lock is 0 cubic feet per second (cfs). There could still be some flow at that lock, according to the U.S. Army Corps of Engineers, but with the lock at Port Mayaca closed, all flow from the St. Lucie canal through the St. Lucie lock is from local basin runoff. The St. Lucie lock is 23.9 miles from the Port Mayaca lock. Flow at the Port Mayaca lock is gravity driven. When the lake level is lower than the water level in the canal, if the Port Mayaca lock water control structures are open, water from the St. Lucie Canal can backflow into the lake.
Lake Okeechobee was at about 11.90 feet on March 29, 2019, the day this photo was taken. In the background is the pier at Clif Betts Jr. Lakeside Recreation at the top of Lake Okeechobee. Special to the Lake Okeechobee News.
As of March 30, flow from the lake to the Caloosahatchee has been reduced to an average of 1,000 cubic feet per second (cfs) as measured at the Franklin Lock. The Franklin Lock is 43.4 miles from the Moore Haven lock. Some flow to the Franklin Lock is local basin runoff that drains directly into the river.
Lee County officials have asked the corps for a flow of 1,000 cfs, measured at the Franklin Lock, during the dry season to benefit the estuaries, which need some freshwater flow, and to prevent salt water intrusion in the river.
The South Florida Water Management District minimum allocation for the Caloosahatchee River the dry season is 400 cfs. The additional flow has been requested by the coastal communities.
“We’ve been successful in lowering the lake this year, which should help regenerate some of the submerged aquatic vegetation, make it easier to do controlled burns, and may help reduce releases during the hot summer months when algae is most likely to bloom,” said Lt. Col. Jennifer Reynolds, Deputy Commander for south Florida. “Drier conditions and the windy weather we’ve been experiencing lately means that evapotranspiration has increased, and the lake has receded more than three-quarters of a foot within the past month. As a result, we have adjusted the flows to the St. Lucie down to zero this week.”
Monday’s lake stage is 11.90 feet NGVD. During the past week, lake levels were reduced by 0.24 feet, with an overall 0.86 foot reduction in the past 30 days. The Corps will continue to monitor conditions and adjust flows as necessary. Any changes in flows to the estuaries will be announced to the public.
Despite releases, east, west and south, most of the water that leaves Lake Okeechobee does so through evaporation into the air and percolation into the aquifer.
The natural ecosystem of Lake Okeechobee needs periods of lower lake levels, which allow the vegetation around the lake’s littoral zone to regrow. That vegetation provides critical spawning areas for fish, and cover for young fish. Anglers who understand the lake’s ecosystem welcome droughts as part of the natural process that keeps the fisheries healthy.
Ideally, Lake Okeechobee should never rise above 15.5 feet and should drop to about 12 feet most dry seasons,” states “Lake Okeechobee: A Synthesis of Information and Recommendations for its Restoration,” by P.N. Gray, C.J. Farrell, M.L. Kraus and A.H. Gromnicki. The study, published in 2005 by Audubon of Florida, was funded in part by the Batchelor Foundation and by the Everglades Foundation.
Webster’s Dictionary explains that evaporation is the movement of water to the air from sources such as the soil, canopy interception and water bodies. Evapotranspiration is the sum of evaporation and plant transpiration from the Earth’s land and ocean surface to the atmosphere.
Scientists use different models to estimate how much water evaporates from the surface of the big lake. On average, they estimate Lake Okeechobee drops by between 4.3 feet and more than 5 feet each year due to the movement of water into the air.
The 2005 Audubon study estimated water loss to evaporation at around 5 feet per year — usually higher during dry periods.
“In an average year, Lake Okeechobee receives enough rain and inflow to raise it about 7.5 feet (note: this discussion of the water budget uses rounded numbers for simplicity).
This water does not come in at once, but is spread over the entire year, thus the lake does not actually rise 7.5 feet each year. Roughly 5 feet of this water will evaporate, leaving 2.5 feet of water in the lake. On average, irrigation and municipal demands use 1 to 2 feet of lake water per year, leaving an “excess” of about one foot of water that must be discharged,” the 2005 Audubon report explains.
During the dry season, when rainfall does not replace the water lost to evaporation, the lake level falls. During the wet season, when water comes in faster than it can go out, the lake level rises.
