In North Charleston, South Carolina last Saturday, the 1,300-foot wide Ashley River was looking slow and lazy as it drifted past the Cooper-Miner Wedding in progress on its left bank. I was thinking of the much smaller Chattahoochee above Helen, splashing excitedly over the rocks in its path. Rivers like the Ashley provide a more tranquil background for weddings. But they actually aren’t slower.
River velocity can be measured precisely with a $1,500 professional instrument. It looks like a 5-inch toy submarine at the end of a long stick. When it’s lowered into the water, the current spins its little propeller, which in turn drives an electronic counter at the other end of a long cable. A computer chip converts the revolutions into feet or inches per second. At low tide, the Ashley tends to move at about 2 feet per second. Farther south, the Savannah River reaches a similar speed, peaking occasionally at 3 feet per second. For those more used to the car speedometer, that’s about 2 miles per hour, or equivalent to a leisurely walk.
A burbling mountain stream looks quicker at first sight. But measuring its velocity will in most cases come up as only a few inches per second. When we look at a mountain creek, we tend to be closer to the water than while observing a large coastal plain river.
The creek carries much less water, so more of it is touching the rough channel walls, where friction slows it down. Splashing over rocks and branches, some of the water even moves back against the general direction of the stream.
Down in the coastal plain, the great amounts of water in the Cooper, Ashley, Savannah or Altamaha River move as if they were in giant pipeline, with no obstacles in the way.
On the last few miles of their journey, the coastal rivers slow down and even reverse direction approximately every 12 hours. That happens when the rising tide pushes ocean water up the channel. If you’re watching patiently, you can see the flow coming upstream as a small wave, called a tidal bore. The gentle tidal bore of a Georgia or South Carolina river measures just a few inches.
On Brazil’s Amazon River though, the Pororoca tidal bore is so tall that it has its own name, sending several feet of water upstream.
Rudi Kiefer, Ph.D., is a professor at Brenau University, teaching physical and health sciences on Brenau’s Georgia campuses and in China. His column appears Sundays and at gainesvilletimes.com.