The St. Johns River is the longest river in Florida at 310 miles (500 km) long. The drop in elevation from headwaters to mouth is less than 30 feet (9 m). Numerous lakes are formed by the river or flow into it, but as a river its widest point is nearly 3 miles (5 km) across. The narrowest point is in the headwaters, an unnavigable marsh in Indian River County. The St. Johns drainage basin of 8,840 square miles (22,900 km2) includes some of Florida’s major wetlands. It is separated into three major basins and two associated watersheds for Lake George and the Ocklawaha River.