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 310 miles (500 km) long, it winds through or borders twelve counties, three of which are the state's largest. The drop in elevation from the headwaters to the mouth is less than 30 feet (9.1 m); like most Florida waterways, the St. Johns has a very slow flow rate at a third of a mile an hour (0.5 km/h), and is often described as "lazy". It is one of a small number of rivers in the United States to run north. Numerous lakes are formed by the river or flow into it, but as a river its widest point is nearly 3 miles (4.8 km) 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.