The Ortona lock is actually located 7 miles upstream …
Of the location of the old Ft Thompson falls.
The Caloosahatchee River is estimated to have dropped 5-10 feet in elevation over that one mile stretch, which was more a series of rapids than a true falls. It was of regional significance as a place to cross the river too, in the day before bridges, during cattle drives for the early European settlers and for Native Americans before that.
Ft Thompson falls backed up Lake Flirt behind it.
The modern-day structure is located at the upstream end of that now dried-up lake bed.