The St. Johns is the longest river in Florida - 310 miles long. It is one of the few rivers in the United States that flows north.
The land area that drains into a water body is called a drainage basin - also called a watershed. The St. Johns is divided into three drainage basins.
Because the river flows north, the upper basin is the area to the south that forms its marshy headwaters.
The middle basin is the area in central Florida where the river widens forming lakes Harney, Jesup. Monroe and George.
The lower basin is the area in Northeast Florida from Putnam County to the river's mouth in Duval County.
The source of the river, or headwaters, is a large marshy area in Indian River County It flows north and turns eastward at Jacksonville to its mouth in the Atlantic Ocean.
The width of the river varies. It is a broad marsh at its headwaters and averages more than two miles in width between Palatka and Jacksonville.
It widens to form large lakes in central Florida.
The total drop of the river from its source in swamps south of Melbourne to its mouth in the Atlantic near Jacksonville is less than 30 feet, or about one inch per mile, making it one of the "laziest" rivers in the world.
Because the river flows slowly, it is difficult to flush pollutants.
Major pollution sources include discharges from wastewater treatment plants and runoff from urban and agricultural areas after it rains.
This runoff carries pesticides and other pollutants into streams that lead to the river. Pollution is concentrated around urban areas.
Saltwater enters the river at its mouth in Jacksonville. In periods of low water, tides may cause a reverse flow as far south as Lake Monroe - 161 miles upstream from the rivers mouth.
Major tributaries, or smaller streams and rivers that flow into the St. Johns River include the Wekiva River, Econlockhatchee River and the Ocklawaha River.
The St. Johns basin is actually an ancient intracoastal lagoon system. As sea levels dropped barrier islands became an obstacle that prevented water from flowing east to the ocean.
Instead, the water collected in the flat valley and slowly meandered northward for about 300 miles. This formed the St. Johns River.