The St. Johns River is the longest river in the U.S. state of Florida and it is the most significant one for commercial and recreational use. At 310 miles (500 km) long, it flows north and winds through or borders twelve counties. The drop in elevation from headwaters to mouth is less than 30 feet (9 m); like most Florida waterways, the St. Johns has a very slow flow speed of 0.3 mph (0.13 m/s), and is often described as "lazy".

Florida is a state in the Southeastern region of the United States. It borders the Gulf of Mexico to the west, Alabama to the northwest, Georgia to the north, the Atlantic Ocean to the east; and the Straits of Florida and Cuba to the south. About two-thirds of Florida occupies a peninsula between the Gulf of Mexico and the Atlantic Ocean. It has the longest coastline in the contiguous United States, spanning approximately 1,350 miles (2,170 km), not including its many barrier islands. It is the only state that borders both the Gulf of Mexico and the Atlantic Ocean. With a population of over 21 million, it is the third-most populous state in the United States and ranks eighth in population density as of 2020. Florida spans 65,758 square miles (170,310 km2), ranking 22nd in area among the states. The Miami metropolitan area, anchored by the cities of Miami, Fort Lauderdale, and West Palm Beach, is the state's largest metropolitan area, with a population of 6.138 million; the most populous city is Jacksonville. Florida's other major population centers include Tampa Bay, Orlando, Cape Coral, and the state capital of Tallahassee.