Century-old canals ripped open Florida's southern cape and ushered in rising seas. Now there's a plan to fix them
Motoring across Lake Ingraham under shorebirds flying in a synchronized swirl that birders call a murmuration, it’s hard to imagine that at one time Henry Flagler and others hatched grand plans to develop Cape Sable.
The shore is a remote fortress of mangroves, interrupted only by a handful of canals. Beyond, sawgrass marshes stretch for miles.
Hacked by hand and excavated by a steam-powered dredger in the 1920s and 30s, the 15-foot wide canals were intended to drain the vast marshes and make way for farms and cattle. Palm trees planted by early homesteaders still mark where they intended to defy the mosquitos and the muck.
Ultimately, life on the cape proved too harsh. The settlement never grew beyond a remote outpost.
But the canals remained, growing wider and wider by the years — to five times their original width — exposing the freshwater marshes to saltwater and rising seas.
In a 2016 report, the National Park Service called the result an “ecological collapse.” The wetlands have been replaced by stretches of open water, often too deep for even mangroves to take root and stabilize the coast. The canals have also allowed mud from the collapsing peat soil to flow back out to Lake Ingraham, the nation’s southernmost lake. Sprawling mud flats now fill the lake.
“When I measured [the Raulerson Canal] a year and a half ago, it was sixty feet and now it's more than 70,” said Audubon Florida biologist Jerry Lorenz, who’s been measuring the canals since 1989 and providing a steady stream of data to Everglades National Park. “That's a disaster.”
Now, a unique partnerships between the park, Audubon, Ducks Unlimited and state and federal wildlife agencies is close to finally damming the Raulerson, the last of the unplugged canals and by far the largest. It’s a project that has mostly escaped notice in the shadow of the massive $23 billion Everglades restoration project. But as South Florida’s first line of defense against sea rise and hurricanes, scientists say protecting marshes that stand as the gateway to the larger Everglades is critical.
“Before there would have been a high ridge that would have kept the saltwater mostly on the south side and freshwater on the north side,” Amy Renshaw, a park scientist monitoring the impacts of the canals, said one morning last month during a visit. “By putting the canals in that ridge, it no longer functions.”
Beyond the punctured ridge — named the buttonwood embankment after the sturdy white mangroves that help keep the shore stable — marshes are collapsing. In satellite images, it’s easy to see the expanding areas of open water that have replaced the dense sawgrass.
“Every tidal cycle flushes huge volumes of saltwater back into the brackish areas,” said Tylan Dean, the chief biologist for Everglades and Dry Tortugas national parks. “The net effect is that the more brackish wetlands behind the buttonwood embankment are just degrading.”
The effect was so compelling that when University of Miami geologist Hal Wanless began studying the area in the 1980s, he cited the cape as evidence of the devastating impacts from rising seas propelled by climate change and the potential for “instantaneous response.” Wanless noted that they’ve also had a cascading effect, causing other tidal creeks to form and hasten the march of saltwater inland.
“Cape Sable is now in a phase of dramatic evolution,” he wrote.
As early as the 1950s, not long after the park formally opened, wildlife managers began worrying about the impacts from the two main canals, the East Cape and Homestead. They were originally dug only 15 to 20 feet wide, a feat given conditions at the time.
“Even in the winter it was plenty hot most of the time,” Lawrence Will wrote in an account of working on the canals in 1922. “The dense growth shut off any cooling breeze, so [workers] were soaked in sweat from dawn till dark. The mosquitoes hovered around them in a constant swarm. To add to their dis-comfort, some areas were covered with water in which they must work for days on end.”
At the turn of the century, about 50 families lived at Flamingo, Will wrote. But by the time Will arrived, “the fortunes of Flamingo were at a low ebb.”
Hurricanes and flooding had left soil too salty to farm, so settlers harvested the mangroves: black for furniture, red for dye and buttonwood to supply a steady demand for coal from Key West, Will wrote.
Lorenz first visited the area to study roseate spoonbills and noticed that the canals were fraying the coast. The sediment washing out was also packed with nutrients. In 1992, when a massive algae bloom erupted in Florida Bay, creating a smelly dead zone that took years to recover, scientists suspect the nutrient run-off flowing from the canals helped trigger it.
Plugs were installed in the canals over the years, but the intense tidal pressure and repeated storms washed them away. Sea rise has accelerated from a leisurely 1 to 2 inches per century over the last 2,000 years to six times faster since the 1930s, the National Park Service reported.
“The 9-inch rise in sea level since 1930 has destabilized all of the Cape Sable’s coastal and wetlands environment,” the 2016 report said.
Since about 2014, better constructed plugs have been installed in the smaller canals. That left the largest and westernmost canal, the Raulerson, named for the brothers who once farmed the area.
Motoring up the canal last month, Renshaw, the park scientist, pointed to a monitoring station installed by the U.S. Geological Survey in 2017. The station is a simple plank made of two boards stretching over the water. Because the canal keeps getting wider, they have to keep extending it, she said.
With a sturdier dam in place — pilings sunk in place to withstand the pounding storm surges and tides — the widening should begin to slow. Some sediment could begin to fill the canals, Dean said, but it’s not clear how long that would take or even if it will happen. With more freshwater flowing into the park and down Shark River, the hope is the sawgrass rebounds.
With Ducks Unlimited managing the construction of the project and Audubon supplying the scientific data, the project should cost between $5 and $7 million.
The construction will be completed in winter months, when endangered crocs aren't nesting and workers "won't be carried off by bugs," said Ducks Unlimited biologist Jeff Beal.
Ducks intends to have plan and permits in place by November, said Ducks engineer Billy Webster.
Back on Lake Ingraham, it’s easy to see why Flagler and early homesteaders would be lured to the cape. Migrating shorebirds confirm their early ambitions. Every year, piping plovers, red knots and a host of others stop here to refuel on migrations that take them from South America to the Arctic.
“Yep, it is a beautiful place,” Dean said. “It's a little hard to get to, but it's worth coming out here if you can manage it.”