The St Johns: River or Estuary?


Florida’s St Johns River is one of the few rivers in North America that flows north…that is, when it’s not flowing south.  Confused?  No worries—so is the river.  You see, Florida’s longest river (310 miles) reverses flow twice a day with high tide.  Quite often this reverse flow extends more than 160 miles upriver!

Old habits die hard for this remnant of an ancient intracoastal lagoon system.  Even when it’s acting like a river, it’s one of the laziest rivers in the world.  Why is it so lazy?  And why does it flow north?  In a word: topography.  You see, its headwaters reside at less than 30 feet above sea level, meaning the river drops less than 30 feet over the course of its 310 mile journey to the sea—less than 1 inch per mile.

Water runs downhill, right?  It always seeks the lowest point, which is typically sea level.  The steeper the drop, the faster it flows.  For instance, as water first plunges over a waterfall it’s travelling at just under 22 miles per hour (9.81 meters per second).  The farther it drops the faster it goes as gravity causes it to accelerate.  This is as fast as water flows naturally.

Waterfalls by definition are vertical drops—the water is in free-fall.  When the angle of descent is only slightly less, you have a cascade.  Reduce the angle of descent even more and you have rapids.  When that angle of descent is reduced to less than an inch per mile the water is barely moving, like 0.3 mph (0.13 meters/second).  By comparison, humans walk about 3 mph (5 kilometers/hour).  In other words, you walk about 10 times faster than the St Johns River flows!

So even at low tide and with adequate rain, when the St Johns is acting like a river, the flow is nearly imperceptible.  This makes it ideal for paddle sports, as there’s never an against-the-current battle to fight.  It’s very much like paddling in an elongated lake, quite similar to the Indian River Lagoon (America’s most diverse estuary).  So don’t plan on the river assisting your downstream paddling…nor impeding your upstream paddling.

The St Johns River is significant enough to be 1 of only 14 rivers in the entire country to be designated an American Heritage River.  Four 1st magnitude springs feed this river, along with many other rich tributaries, like the Wekiva River, which is designated by the federal government as a  National Wild and Scenic River.

While the St Johns is a river, it acts very much like an estuary…echoes of its former self.  There are many locations to access and enjoy this amazing river and the lakes it forms along the way.  We’ll be exploring them to bring you the best of this Florida Natural Wonder.  For starters, a St Johns River Cruise launches from Blue Spring State Park.  I recently took this roughly 2-hour tour and am happy to report that it was both entertaining and informative.  Check out my photos, videos and WonderMap of the River Cruise—then plan to add this to your Florida Bucket List for Nature Lovers.

 

References & More Information:

http://en.wikipedia.org/wiki/St._Johns_River

http://sjrwmd.net/stjohnsriver/

http://sjrivercruises.com/

http://en.wikipedia.org/wiki/Standard_gravity

 

 

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