Well the fact Jacksonville is right there in the concave with Georgia and S. Carolina ....and dirtboy shared about that area could well have very serious storm surges I suspect St.Johns River would indeed increase substantially. The water has to go somewhere and the Bay certainly is right there to do just that.
Too close for comfort with family members there..though high and dry you just can’t tell with this beast rolling!
Rivers have been known to flow backwards for a period of time due to wind and storm surge, so flooding could back up inland a ways.
I’m about a mile from the river. We didn’t flood for Matthew but supposedly we are in a “500 year flood” zone.