The St. Lucie and Caloosahatchee estuaries have been extensively changed from the systems designed by nature.
During an online public webinar on May 28, Patti Gorman of the South Florida Water Management District, explained how development and flood control efforts have drastically changed the natural estuaries.
Historically, the St. Lucie River was a freshwater river with no connection to the Atlantic Ocean and no connection to Lake Okeechobee, she explained. The inlet was initially opened by businessmen in 1898 for boat traffic.
“The St. Lucie, for most of its history, was mostly a freshwater system,” she said. “As things got developed and the inlet was dug and hardened, it because a saltier situation.
“That was before the major watershed development was done,” she said. As development continued, the salinity levels changed again.
In addition, the C-44 Canal was dug to connect Lake Okeechobee to the St. Lucie River, as part of the Lake Okeechobee Waterway, both for navigation and for drainage.
“We can’t restore back (to the estuary’s natural state) because we are not going to close the inlet,” she said. “We’re not going to move everybody out of the watershed.”
Water managers try to balance a healthy ecosystem with current constraints, explained Ms. Gorman.
She said about 60 to 70 percent of the water that enters the St. Lucie is from local basin runoff.
High inflows from either the watershed or the lake bring in water with dissolved, suspended matter which reduces light to the sea grasses. With low flow of freshwater, sometimes the salinities get too high, she added. Salinity levels that are too high or two low can be harmful to the estuaries.
The Calooshatchee River’s estuary was also changed by development and flood control, she said.
Originally, the Caloosahatchee River started at Lake Flirt, near Fort Thompson. During the wet season, water flowed through a series of marshes and smaller lakes (Lake Hicpochee, a Lettuce Lake and Bonnet Lake) from Lake Okeechobee to Lake Flirt then over a small waterfall into the river.
The lake was first connected to the river in 1881 by Hamilton Disston, to allow navigation from Lake Kissimmee to Fort Myers. It was later straightened and deepened to improve navigation and for flood control.
During the dry season the river, as designed by nature, received no flow from Lake Okeechobee. However, Ms. Gorman explained, the local basin drained much more slowly into the river before flood control, so the river originally had some freshwater flow from the basin during the dry season. Now the river depends on freshwater flow from the lake during the dry season to prevent saltwater intrusion.
The Caloosahatchee River is a highly managed system with locks and dams, not a free-flowing system, she said.
The water enters the river at the S-77 structure at Moore Haven. From there it flows to the S-78 structure at Ortona, then the S-79 (the Franklin Lock). The area from the S-78 to the S-79 is freshwater, she said. “From the S-79 out, we try to retain the gradient for marine system.
“In some ways, we know a lot about the estuaries. In other ways, it gets very complicated to manage these things,” Ms. Gorman said.
If there is too little freshwater flow coming in during the dry season, the salinity gets too high for some of the marine organisms, mangroves — which like the higher salinity — start moving upstream and silty sand and mucky sand can accumulate in the middle estuaries.
“We’re trying to balance this entire estuary system which goes from a purely freshwater system to a brackish area to an ocean system,” she said.
She said they have found that if the freshwater flow at the S-79 falls below 450 cubic feet per second (cfs), the salinity level gets too high for some species in the estuary.
If the flow rises above 2,800 cfs, the salinity levels drop too low for some marine species in the estuary.
“We are trying to maintain balance for the static and dynamic habitat,” she said.
“It’s all about the quantity, quality and timing of fresh water,” she said.
Too much water during the wet season can hurt the estuaries. Too low flow in the dry season causes saltwater intrusion and damage to some marine organisms.
Both estuaries have both of these problems, she said.