What the lower Cape Fear’s perishing cypress trees are trying to tell us
Story and Photographs by Virginia Holman
As visitors to Wilmington arrive along the I-74 corridor or down the Martin Luther King Jr. Parkway from the airport, they are greeted by a haunting scene: hundreds of weather-silvered trees — tupelo, cypress, gum — some festooned with long swaying beards of Spanish moss. It’s a melancholy sight. There’s an eerie beauty to these “ghost forests,” but it’s the same sort of beauty one finds in old cemeteries where lichen decorates nameless weathered headstones. And though every living thing is said to have a natural season to flourish and to die, most of these trees were killed before their time. The cause is human activity that led to saltwater intrusion in our inland tidal creeks.
The Cape Fear River is the only river in North Carolina that flows directly into the Atlantic Ocean, and if you’ve ever been on the river when the tide has recently turned, you may have been privy to an amazing sight: a tideline of brilliant emerald ocean water pushing against the sweet tea-colored Cape Fear. This line in the water, according to Sandie Cecelski, director of Ashley High School’s Marine Science Academy, is part of a “salt wedge.” When saltwater and freshwater collide, they don’t mix easily. Saltwater is more dense than freshwater, so when the two meet, as they do in near the mouth of the Cape Fear, the freshwater floats above the saltwater, so the saltwater must flow below. To get an idea of what this looks like beneath the river’s surface, think of a rectangle that’s bisected diagonally, forming two wedges. The freshwater is on top and the saltwater on the bottom. The highest point of the diagonal is that dramatic dividing line you see on the water. The first time I kayaked along a salt wedge tideline, the demarcation line appeared to simmer. I then saw it was a bit of a fish trap — dolphins and a racket of terns arrived to feast on a menhaden buffet.
Farther up the lower Cape Fear, in the estuarine marshes between Bald Head and Wilmington, saltwater and freshwater mix well and provide critical nursery habitats for North Carolina’s fisheries. (Not incidentally, recreational and commercial fishing are important economic drivers for southeastern North Carolina.) Slightly inland, still intact freshwater swamp forests and wetlands can be seen in larger tracts of preserved land like Carolina Beach State Park. These wetlands house salt-intolerant trees such as cypress, swamp maple and tupelo. When saltwater intrusion begins, it’s often these freshwater wetlands that first become “ghost forests,” a somber warning sign of an environment under tremendous stress.
Living swamp forests and estuaries serve important functions. Cecelski explains that just as our saltwater and estuarine marshes protect inland areas from storm surge and filter storm water contaminants, our riparian swamp forests “act like a sponge, collecting, filtering, and breaking down many contaminants in surface runoff.” These pollutants would otherwise flow directly into the river and, in the case of the Cape Fear, into the ocean with the outgoing tide.
The past 50 years have seen the local landscape alter at a rapid pace: Swamp forest wetlands have been filled to build roads, parking lots, homes and shopping centers. To avoid flooding the property built on former freshwater wetlands, we push the storm water that collects on these impervious surfaces to a network of pipes and ditches that lead to the water where we fish and swim. Storm water once filtered by those wetlands is now laden with pollutants like pesticides, herbicides and animal feces.
Dr. Larry Cahoon, a professor in UNCW’s Biology and Marine Biology Department, has witnessed the ruin of vast swaths of the county’s wetlands. “Much of New Hanover County is freshwater wetlands, or was; now it’s Pine Valley and places like that. I remember flying over the county in a small plane back in the early 1980s, and I was amazed at how much sunlight was reflected off standing water throughout much of the county. Much of it was forested wetland, especially the northeast and southern part of the county, where they’ve subsequently built all those developments on South College Road.”
Rapid development also comes with a steep price tag. From 1999 to 2016, approximately $42 million was spent on storm water drainage mitigation in the city of Wilmington alone. The design, permitting and implementation of Wilmington’s currently planned storm water capital improvement projects from 2017 to 2020 are projected to total an additional $32 million.
“The sentinel of the freshwater wetland is the cypress tree,” says Cahoon. “They are the first to die when a freshwater wetland becomes salinized.” Cahoon notes that after 2000, “We saw a surge in cypress tree die-offs in many of the creeks” of the Cape Fear — including areas like Smith Creek, which can be seen from Martin Luther King Jr. Parkway — after the shipping channel was deepened 4 feet from mouth to port. “Those trees grew when the marsh was freshwater and they died as the marsh became salinized, so they are a good indicator of change over time.”
Professor Cahoon gives me a basic primer on how deepening the river channel causes an increase in salinization. “A deeper channel means the volume of sea water that can enter the river increases. Because we have tides, the water sloshes up and down the river, and one of the effects that you can have is an amplification of the tidal range at the top of the estuary. You’ve got all this water sloshing upstream toward downtown Wilmington, and when it gets there, it runs into all these small river channels and creeks. We have seen a significant increase in the tidal amplitude at high tide in downtown Wilmington.”
Though it seems counterintuitive, droughts also allow for an increase of intrusion. When river flow drops due to drought, he explains, the freshwater volume is replaced by salty ocean water. Cahoon says there is reliable data that shows during major drought events, like the one we had in 2007, salinity levels increase as far up as the confluence of the Cape Fear River and the Black River, which is several miles downstream from Lock and Dam Number One. “Most of the time,” he says, “you don’t have a lot of saltwater much above the northern edge of the city since salinity levels vary with the river flow.” But many areas have detectable salinity much of the time. “You can taste the river by the Battleship — though you may not want to — and you’ll find it’s slightly salty.”
In addition, a recent study published in the September 2016 issue of “Geophysical Research Letters” indicated that the deepening of the Cape Fear River has dramatically increased the effects of tides and storm surge along the riverbank. Models indicate that storm surge from a category five hurricane in the 19th century, when the shipping channels were half their current depth, would have been around 12 feet. Currently, it is estimated at 18 feet.
Saltwater intrusion affects more than the natural world: It affects our infrastructure. “Along the coast, like in Carolina Beach, if you dig a hole, you are going to hit water sooner at high tide than low. The high tide actually raises and lowers the groundwater levels, so when you get a high tide there, it affects the groundwater levels. Our sewer pipes are in the ground,” says Cahoon. “When large tidal events like spring and king tides occur, the infrastructure in the ground is inundated with saltwater.” The same is true of storm surge. “Sewage is pretty corrosive as is, and saltwater makes it worse.”
Cahoon points out a few additional salient facts. “Saltwater contains sodium chloride as well as sulfate, which is a source of oxygen for anaerobic bacteria. So, when seawater enters a sewage system, the bacteria will use that sulfate as a source of oxygen. The byproduct is hydrogen sulfide, which smells like rotten eggs. So, what’s wrong with that? Hydrogen sulfide is very reactive, and it reacts with metal. It forms metallic sulfides, so the metal parts are degraded. In addition, sulfide reacts with the calcium in concrete and it actually will degrade the concrete they use to make manholes. If your manholes are exposed long enough, they will degrade and come apart.
“Some of our sewer pipes are built from materials that are not that tough. In New Hanover County we used a lot of cast iron pipes, which disintegrate. The countywide sewer system was installed in the late 1980s.” The average cost to install a sewer system in United States, says Cahoon, is a staggering million dollars per mile. “We have over 900 miles of sewer pipes in New Hanover County.”
Cahoon says that sea level rise will exacerbate all of these issues. “And whether you believe in sea level rise or not,” he laughs, “it believes in you, and it’s coming for you.”
It’s unclear what we can do to try to slow the effects of saltwater intrusion. After all, it’s hard to quickly recreate a wooded freshwater wetland once it’s dead or gone. What’s clear is that those ghostly cypress trees are telling us something, and we’d do well to listen.
Author Virginia Holman, a regular Salt contributor, teaches in the creative writing department at UNC Wilmington.