text: what is the largest river in florida???? ok what is the largest river in floirda the largest river in the state of florida is the st. johns river which is 310 miles or 500 kilometers long.  it is the longest river in the united states that flows north instead of south.  it is also considered on of the laziest rivers in the world since there is only a 30 foot elevation change from indian river county to the atlantic ocean.  it is not uncommon to find dolphins and manatees swimming the waters of this great lake.  for a good bit more info this river click on the site below.