Named for the white, almost skeletal trunks of dead trees, so-called “ghost forests” are typically caused by development or climate change.

In the St. Johns River, they’re markers of rising seas and surging salt levels.

The current state of the river is anything but salty. Jacksonville University’s Marine Science Research Institute Executive Director Dr. Quinton White said Hurricane Irma profoundly changed salinity levels in the St. Johns, at least in the short term.

"I’ve never seen anything like it,” he said. “I’ve never seen it run this fresh, this long. It’s very, very unusual.”

White, who has studied the river for more than 40 years said straight ocean water is measured at a salinity rate of 35 parts per thousand. The river near Jacksonville University – about 18 miles west of the Atlantic – has salinity in the range of 15 to 18 parts per thousand.

After Irma, that number fell to almost zero.

“It dropped to almost zero for weeks and weeks and weeks,” White said.

The St. Johns River Water Management District calculated 2.2 trillion gallons of water fell over the 18-county watershed, enough to cover 6.7 million football fields in a foot of water.

Because the river is so slow-moving and flat, that rainwater – even 11 weeks later -- is still trying to push its way out

“That amount of water is taking quite literally months to run off,” White said.

The freshwater influx is actually the opposite of the river’s main problem. In recent years, ocean water has pushed farther upstream, harming aquatic life and causing unmistakable changes to cypress forests along its banks. Ghost forests are now common sights in the Beauclerc and Mandarin areas, particularly along Julington, Goodby’s and Christopher creeks; areas where natural riverbanks still exist.

“It’s ironic that we’re having this conversation,” White said. “When the bigger concern is activity which is making the salinity go up.”

St. Johns Riverkeeper Executive Director Jimmy Orth said the increase in salinity is a direct result of human impacts, including withdrawals from the aquifer that deplete the river's flow, and dredging projects that allow more saltwater to flow from the ocean.

“We’ve altered the St. Johns dramatically over time,” Orth said. “The mouth of St. Johns at one time was probably 14 feet deep, and now we’ve dredged it down to 40 feet.”

Orth said salinity will only increase if and when JaxPort dredges the river another 7 to 10 feet to accommodate larger cargo vessels. (The port has begun the initial phase of the dredging without a funding source for the full 11-mile dredge.)

“In some parts, it’s more like an extension of ocean,” he said of the area between Blount Island and the ocean. "And it’s just getting worse."

The freshwater flow might give trees a temporary respite, but White said it will be short lived.

“We’re going to see salinity creep back up,” he said. “And it will continue marching south."