The knee jerk reaction is releases from the Lake.
|Annual discharge down the Caloosahatchee|
as measured at the S-79 WP Franklin structure
But don't forget that the Caloosahatchee is a natural (and human enhanced) drain for the surrounding watershed, too. Prior to Ft. Thompson Falls being blasted, the oxbows being straightened and feeder canals connected to the main channel, that contribution too was probably a lot less. Meanwhile, while direct contributions from the Lake Okeechobee did not exist, it was a lake (named Flirt) that on the upstream side of the falls formed the original headwater source of the heavily-oxbowed river. And the watershed flow it did get was more groundwater than direct surface water (i.e. canal) flows.
Suffice it to say things have changed.
But it's still a two-source river as shown above.