The St. Johns River is the longest river in the U.S. state of Florida and its most significant for commercial and recreational use. At long, it winds through or borders twelve counties, three of which are the state's largest. The drop in elevation from headwaters to mouth is less than ; like most Florida waterways, the St. Johns has a very low flow rate and is often described as "lazy". It is believed to be one of the few rivers that flow north, although there are hundreds of the kind worldwide. Numerous lakes are formed by the river or flow into it, but as a river its widest point is nearly across. The narrowest point is in the headwaters, an unnavigable marsh in Indian River County. In all, 3.5 million people live within the various watersheds that feed into the St. Johns River.