page_title: St. Johns River
section_title: Flow rates and water quality
hierarchical_section_title: St. Johns River / Formation and hydrology / Flow rates and water quality
caption_attribution_description: English: Fog on the St. Johns River (Florida), taken at the Shands Bridge on the Duval/St. Johns County side, 9 am
context_page_description: The St. Johns River is the longest river in the U.S. state of Florida and its most significant one for commercial and recreational use. At 310 miles long, it flows north and winds through or borders twelve counties. The drop in elevation from headwaters to mouth is less than 30 feet; like most Florida waterways, the St. Johns has a very low flow rate 0.3 mph 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 3 miles 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 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.
context_section_description: The entire river lies within the nearly flat Pamlico terrace, giving it an overall gradient of 0.8 inches (2.0 cm) per mile (km); it is one of the flattest major rivers on the continent. Its proximity to the ocean in the lower basin affects its rise and fall with tides and salinity. Tides regularly affect water levels as far south as Lake George; when combined with extreme winds, the river's tidal effects can extend to Lake Monroe 161 miles (259 km) away and have on occasion reached Lake Harney. Tides typically raise the river level about 1.2 feet (0.37 m) at Jacksonville, decreasing some to 0.7 feet (0.21 m) at Orange Park where the river widens, and increasing back to 1.2 feet (0.37 m) at Palatka as it narrows. As a result of tidal effects, discharge measurements in the lower basin are often inaccurate. However, the estimated rate of discharge between the Ocklawaha River and the center of Jacksonville ranges from 4,000 to 8,300 cubic feet (110 to 240 m³) per second. The nontidal discharge at the mouth at Mayport averages 15,000 cubic feet (420 m³) per second, but with tides it exceeds 50,000 cubic feet (1,400 m³) per second, and following heavy rains combined with tides can top 150,000 cubic feet (4,200 m³) per second. Farther upriver, the discharge rate ranges from 1,030 cubic feet (29 m³) per second near Lake Poinsett to 2,850 cubic feet (81 m³) per second near DeLand. The confluence of numerous springs, the Econlockhatchee River, and the Wekiva River causes the average discharge to increase by 940 cubic feet (27 m³) per second between Lake Harney and DeLand, representing the greatest annual average increase of streamflow along the St. Johns.
  As distance between the mouth of the St. Johns and the middle and upper basins increases, the salinity in the river gradually decreases. Marine water measures at 35 parts per thousand (ppt) or more while fresh water measures below 2 ppt. What ranges in between is characterized as brackish water. Near the center of Jacksonville, average measures have been collected at 11.40 ppt. Farther south at the Buckman Bridge, joining the south side of Jacksonville to Orange Park, it decreases to 2.9 ppt and falls again to 0.81 ppt at the Shands Bridge near Green Cove Springs.
  Dissolved oxygen in fresh water is measured to indicate the health of plant and animal life. It enters water through the atmosphere and from aquatic plant photosynthesis, and is affected by water pressure and temperature. Rapid decomposition of organic materials will decrease the amount of dissolved oxygen in the river, as will nutrients added to the water artificially by wastewater treatment runoff or drainage from fertilized agricultural fields. The U.S. Environmental Protection Agency and the State of Florida recommend no less than 5 mg of oxygen per liter. Several locations on the St. Johns or its tributaries reported at or below these minimums in the 1990s, including the mouth of the Wekiva River, the St. Johns at the town of Christmas, and in the early 2000s at Blue Spring and Blackwater Creek. Sustained low levels of dissolved oxygen may create algal blooms, which may also cause a further decrease in dissolved oxygen.
  Like all blackwater streams in Florida, the color of most of the St. Johns is black, owing to the tannins in leaf litter and decaying aquatic plants. Spring fed streams, on the other hand, are remarkably clear and visibility is very high, even when the river bottom is dozens of feet below.
caption: Fog on a calm St. Johns River at the Shands Bridge near Green Cove Springs
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