Saltwater Intrusion

Joseph Guiffre

How often do you think about the depth of the St. Johns River? Probably not on a regular basis but the question has been on the minds of many of the people in charge of the City of Jacksonville for a very long time.

JaxPort is one of the most important economic centers in Northeast Florida, bringing vast amounts of cargo through the mouth of the St. Johns River and on to the highways leading out of the city, making Jacksonville a logistical center of the entire country. As it turns out, the logistics business is competitive, and Jacksonville is desperate to compete with other ports in the region like Savannah and Charleston. What business leaders say Jacksonville needs to do to keep up has a lot to do with the depth of our river.

City leaders called upon the Army Corps of Engineers to take on the task of dredging the St. Johns River between Mayport and the Dames Point Bridge. The culmination of over a decade of planning, the $484 million project, expected to be completed this month, will enable JaxPort to allow super-sized cargo ships to arrive from all over the world via the Panama Canal. For many, its completion would appear to be a huge win for the city, and in many ways, it is, but not everyone agrees. The St. Johns Riverkeeper has been at the forefront of fighting this dredging project for years, even filing suit against the Army Corps of Engineers to put a halt to the project in 2017, an effort that did not succeed. The cost of deepening the St. Johns River isn’t just in dollars and cents: It could cost a whole lot more than that.

Among the many environmental concerns raised by the Riverkeeper was the change in saltwater boundary in the river, also known as saltwater intrusion. The salinity of the river dictates everything from what kind of animals live in the water to what kind of plants grow on the shores. Historical records indicate the point of change from saltwater to freshwater in the river in the 1950s was near the current day Acosta Bridge; now it is thought be somewhere in between the Buckman Bridge in Mandarin and the Shands Bridge in Green Cove Springs, nearly 50 miles from the mouth of the river. One of the major species that will be impacted by the change is the Florida manatee that grazes on the submerged aquatic vegetation, or seagrass, that only grows in freshwater environments. Once a safe haven for manatees, the St. Johns River will continue to lose critical areas of seagrass farther and farther upstream. The disappearance of seagrass is happening all over the state due to fertilizer runoff, algae blooms and an uptick in people treading on the delicate plants.

Saltwater intrusion is also a threat to the people who live on banks of the St. Johns River. As the saltwater moves deeper into the river system, freshwater trees like bald cypress will no longer be able to survive and will eventually die off, an effect that is already being observed as far south as Julington Creek. The cypress trees play an important role in preventing erosion along the river banks: Their roots hold the soil on the shore in place, and their massive trunks slow down waves that eat away at the ground underneath bulkheads and eventually make their way onto residents’ properties.

Studies commissioned by the Army Corps of Engineers also show flooding will increase throughout the lower basin of the St. Johns River after the dredging is complete. Floods will have greater strength due to the increased flow of saltwater into the St. Johns and the widening of the channel. Added to the already high waters experienced during hurricanes and powerful storms, the dredging could put millions of dollars of property and possibly thousands of people in harm’s way.

There are some solutions being offered by environmental advocates like the St. Johns Riverkeeper and organizations calling for better green infrastructure like Groundwork Jax. The Riverkeeper, for example, has demanded a plan be initiated to decommission the Rodman Dam (which blocks the Ocklawaha River from entering the St. Johns) in exchange for dropping any continued lawsuits against the deepening of the port. Allowing the Ocklawaha to flow freely will pour hundreds of millions of gallons of freshwater into the St. Johns River system, pushing back against the rising tide of saltwater. Groundwork Jax is working toward the restoration of urban wetland areas like McCoys Creek and Hogans Creek, giving stormwater runoff and future flood waters somewhere to overflow. Both of those proposals are in the early stages of being completed.

The St. Johns River’s beauty and function are strong currents that tie our city together, and it is worthwhile to take a moment to consider the depths of those waters and how we are changing them.