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 long, it flows north and winds through or borders twelve counties. The drop in elevation from headwaters to mouth is less than ; like most Florida waterways, the St. Johns has a very slow flow speed of , and is often described as "lazy".
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. The St. Johns drainage basin of 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, all managed by the St. Johns River Water Management District.
Although Florida was the location of the first permanent European settlement in what would become the United States, much of Florida remained an undeveloped frontier into the 20th century. With the growth of population, the St. Johns, like many Florida rivers, was altered to make way for agricultural and residential centers, suffering severe pollution and redirection that has diminished its ecosystem. The St. Johns, named one of 14 American Heritage Rivers in 1998, was number 6 on a list of America's Ten Most Endangered Rivers in 2008. Restoration efforts are underway for the basins around the St. Johns as Florida's population continues to increase.
Historically, a variety of people have lived on or near the St. Johns, including Paleo-indians, Archaic people, Timucua, Mocama, French, Spanish, and British colonists, Seminoles, slaves and freemen, Florida crackers, land developers, tourists and retirees. It has been the subject of William Bartram's journals, Harriet Beecher Stowe's letters home, and Marjorie Kinnan Rawlings' books. In the year 2000, 3.5 million people lived within the various watersheds that feed into the St. Johns River.
Geography and ecology
Starting in Brevard County and meeting the Atlantic Ocean at Duval County, the St. Johns is Florida's primary commercial and recreational waterway. It flows north from its headwaters, originating in the direction of the Lake Wales Ridge, which is only slightly elevated at above sea level. Because of this low elevation drop, the river has a long backwater. It ebbs and flows with tides that pass through the barrier islands and up the channel. Uniquely, it shares the same regional terrain as the parallel Kissimmee River, although the Kissimmee flows south.
Upper basin
The St. Johns River is separated into three basins and two associated watersheds managed by the St. Johns River Water Management District. Because the river flows in a northerly direction, the upper basin is located in the headwaters of the river at its southernmost point. Indian River County is where the river begins as a network of marshes, at a point west of Vero Beach aptly named the St. Johns Marsh in central Florida. The St. Johns River is a blackwater stream, meaning that it is fed primarily by swamps and marshes lying beneath it; water seeps through the sandy soil and collects in a slight valley. The upper basin measures approximately ; the St. Johns transforms into a navigable waterway in Brevard County. The river touches on the borders of Osceola and Orange Counties, and flows through the southeast tip of Seminole County, transitioning into its middle basin a dozen miles (19 km) or so north of Titusville.
The upper basin of the St. Johns was significantly lowered in the 1920s with the establishment of the Melbourne Tillman drainage project. This drained the St. Johns' headwaters eastward to the Indian River through canals dug across the Ten-Mile Ridge near Palm Bay. As of 2015, these past diversions are being partially reversed through the first phase of the Canal 1 Rediversion project.