The bottom of the Ocmulgee River is rarely visible, but in the sonar image it looks like a gently rippling sheet of pale blue, moonlit clouds.
Several miles upstream, the image changes to resemble big puffy clouds as the river bottom changes from sandy to rocky.
“Some people say it looks like the surface of the moon,” said Thom Litts, who specializes in geographic information systems for the Georgia Department of Natural Resources.
Litts was riding in the front of a johnboat Thursday steered by Adam Kaeser, his partner in developing a new sonar scanning technique for mapping river and stream beds.
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The two were on the Ocmulgee River north of Ga. 83 looking for patches of underwater gravel where the endangered robust redhorse fish might lay their eggs.
A key to understanding any animal species is knowing where it likes to live, breed and search for food — basic habitat information that is elusive in the murky underwater world.
“One of the challenges people that study streams and rivers face is they really can’t put themselves in the environment where the animals live,” said Stephen Golladay, a biologist at the Joseph W. Jones Ecological Center at Ichauway, where Kaeser and Litts refined their technique.
But the sonar mapping method developed by the two researchers uses relatively low-cost equipment to provide habitat information that helps scientists understand where fish and rare mussel species are spending their time, Golladay said.
Kaeser and Litts’ research has been prominently published in the journal of the American Fisheries Society, and the pair has trained other state, federal, private and nonprofit professionals in the U.S. and Canada on how to use the technique.
It starts with a Humminbird “side-scan sonar” unit, which costs about $2,000 and is marketed to professional anglers. The sonar unit is connected to a transducer, a device the size of a small remote control that is mounted on a pipe at the front of the boat.
The transducer sends sound waves in every direction under water. Waves striking rocks, fallen trees, gravel and other underwater objects send back an echo to the transducer. The echoed sound waves are translated into a picture visible instantly on a screen on the boat unit.
“It takes a while to get used to the idea that you’re looking at a sound,” Kaeser said. “Shadows are sound shadows, not light shadows.”
Large rocks can form long, dark shadows on the image because they block sound waves. Rapids and shoals on the river can cause distortions in the picture because the rushing water makes sounds, Kaeser said.
Litts has developed a program that combines the sonar imagery with GIS coordinates and a widely available mapping program to create a map of the river showing all the different types of habitat on the river bottom.
In the past, getting this kind of information required a two-member crew traveling down the river, stopping at different points to run a rope across and then walk, swim, snorkel or scuba dive along the rope to visually inspect the bottom, Kaeser said. Between each of these research points, they would have to estimate what the river bottom might look like.
In their paper, Kaeser and Litts estimated that it would take 10 times as long to map the same distance of stream using this traditional method, and the results would be less certain.
But the pair are still refining their own techniques. Some riverbed surfaces, such as gravel, are hard to identify on the sonar.
That’s why the two were searching for gravel beds they could find visually while the Ocmulgee’s flow is low.
Helps restocking efforts
Several miles north of the Ga. 83 bridge, Kaeser found the first good-quality gravel bed of the day and waded waist deep around its perimeter to record global positioning coordinates. Later, he can return and use the sonar to inspect the same area when the water is deeper, helping him recognize what that gravel looks like on sonar.
Gravel mapping could help the DNR track the success of efforts to stock the endangered robust redhorse in the Ocmulgee River, Kaeser said. The species probably once populated the entire river, but their only natural remnant population is in the Oconee River.
By coincidence, several University of Georgia researchers were also boating the same stretch of the Ocmulgee on Thursday, using electroshocking equipment to look for robust redhorse fish in the area (without much luck).
The surface of the Ocmulgee, clear but very shallow late in a dry September, was scarred all over by the branches of fallen trees. Those that stretched beneath the water appeared as lines in the underwater images.
Blue dragonflies raced alongside the boat as Kaeser struggled with its sputtering motor. “The Humminbird (sonar system) is worth more than the whole boat, by a long shot,” he said with a chuckle. Yet other high-tech mapping approaches use far more expensive technology and require highly trained specialists to operate.
The robust redhorse research is only one of many applications for the sonar. Kaeser said his team was recently awarded federal and state grants totaling $150,000 to map the entire Ocmulgee, Oconee, and Altamaha rivers, plus a portion of the Savannah. This will provide baseline habitat information scientists can later use to identify where habitat is being lost, so they can start figuring out why, Litts said. It could also eventually help the state identify critical habitat for the Atlantic and short-nosed sturgeon. Those species, once common in these rivers, have been nominated to the endangered species list.
In the Coosa River Basin, sonar mapping can be used to identify cobble and boulder river bottom habitats preferred by lake sturgeon. The continent’s largest fish, the rare lake sturgeon is the subject of a large species-restoration project there, Kaeser said.
In the lower Flint River basin, a map of the river bottom could help with future restoration of gulf sturgeon populations and research on rare shoal bass, he said. And Golladay said students at the Jones Center who are studying turtle species found only in the Flint River system have used the maps to identify habitats where radio-tracked turtles rest and feed.
“This is a great resource for people trying to better manage river flows and river resources in Georgia,” Golladay said.
To contact writer S. Heather Duncan, call 744-4225.