St. Johns River by the Numbers: 310 Miles, a Northward Flow and Less Than 30 Feet of Drop

Jacksonville, Fla. - The St. Johns River stretches 310 miles across Florida, flows north instead of south and drops less than 30 feet from its beginning near Vero Beach to the Atlantic Ocean at Jacksonville. It connects lakes, springs, wildlife habitat and communities across a drainage basin covering 8,840 square miles.

Despite its size, the river moves slowly. Its overall flow averages about 0.3 mph, earning it a reputation as a "lazy" river.

From changing salt levels around Jacksonville to tides felt more than 160 miles inland, here is a closer look at the St. Johns River through the numbers.

1. The river stretches 310 miles and flows north

The St. Johns begins in the St. Johns Marsh near Vero Beach in Indian River County, at an elevation of about 30 feet.

From there, it travels north to the Atlantic Ocean near Mayport in Duval County.

Across all 310 miles, the water drops less than 30 feet, about 0.8 inch per mile. That gentle slope helps explain why the river moves so slowly.

2. A watershed covering 8,840 square miles

The river's drainage basin covers about 8,840 square miles. It touches or borders 12 Florida counties, including Brevard, Osceola, Orange, Lake, Volusia, Seminole, Putnam, St. Johns, Clay and Duval.

About 3.5 million people live in the river's watersheds, which shows why the St. Johns matters far beyond Jacksonville.

3. Its widest stretches are near home

The river changes a lot in width as it moves north.

Between Lake George and Palatka, it ranges from about 600 feet to 2,640 feet wide. Between Palatka and Jacksonville, it widens to about 1 to 3 miles across. Its widest point is about 3 miles.

The shipping channel north of Jacksonville is much narrower: 400 to 900 feet wide and 40 feet deep.

St. Johns River: The numbers at a glance

4. Tides push 161 miles inland

Although the St. Johns flows north, Atlantic tides affect it far upstream. Tidal influence reaches about 161 miles inland to Lake Monroe and occasionally to Lake Harney.

At Mayport, average discharge is about 15,000 cubic feet per second. With tides, it can top 50,000. After heavy rain combined with tides, it can reach 150,000 cubic feet per second, 10 times the average.

5. Big lakes, and about 3,500 smaller ones

The upper basin holds about 3,500 lakes. Three big ones stand out:

Lake George is nearly five times the size of Lake Monroe and eight times the size of Lake Harney.

6. Blue Spring pumps more than 64 million gallons a day

Blue Spring releases more than 64 million gallons of water a day, at a constant 72 degrees.

7. Salt levels change a lot around Jacksonville

How salty the river is depends on where you measure it.

Near downtown, the river is a mix of salt and fresh water. By the Shands Bridge, it measures as fresh.

8. Wildlife includes 183 fish species

The St. Johns system supports 183 fish species, including 55 in the main river.

Birds are plentiful too. One study counted about 60,000 birds in the upper basin in a single month.

9. Human history stretches back thousands of years

People lived along the St. Johns 5,000 to 3,000 years ago, during the Middle Archaic period. About 14,000 Timucua lived in the region at the time of European contact.

10. The port shows the river's economic muscle

According to the article's figures, Jacksonville's port handles about 18 million short tons of cargo a year, with a local economic impact of $1.38 billion and about 10,000 jobs.

Seven bridges cross the river in Jacksonville.

Quick Q&A

Does the St. Johns really flow north? Yes. It starts near Vero Beach and empties into the Atlantic at Mayport.

Why is it so slow? It drops less than 30 feet over 310 miles, about 0.8 inch per mile.

How far inland do tides reach? About 161 miles, to Lake Monroe and sometimes Lake Harney.

What the numbers mean for Jacksonville

The St. Johns is more than the stretch of water running through downtown. Its 310 miles tie Jacksonville to a network of lakes, springs and waterways across Florida. Its gentle slope explains its slow pace, and its changing width, flow and salt levels mean the river can look and behave very differently from one spot to the next.

How we did this

Jax Times compiled the measurements, dates, wildlife figures and Jacksonville shipping numbers from Wikipedia's "St. Johns River" article, reviewed Oct. 23, 2026. The figures are the article's, not new field measurements or updated port statistics.