Sonar interpretation-what am I looking at??

Thread starter #1
I must admit I still struggle figuring out what I'm looking at on my Humminbird. I was out today trolling under a bridge and my screen was filled up with little specks. Notice how there's a band of red with a band of blue on top. Sometimes it was just solid blue without the red. What am I looking at?? Small bait, brush, thermocline? I was scouting for a place to night fish for crappie and wanted to see what was down there. Anyone having any luck night fishing for crappie on Lanier? Thanks,



Senior Member
I think that’s some type of interferance not even the bottom looks like a regular sonar image. Did it look like this any place else or was this an isolated incident?
Thread starter #4
The transducer is mounted to the bottom of my trolling motor. That was the only thing running. Even when I took my foot off the pedal it was still there. I’m assuming if it was fish you’d see something more like arches or squiggly lines.
Appears to me you have the “gain” set too high. You should have a clear and defined bottom in chirp mode. On my Garmin I run mine in “auto” mode that way if I run in clear or muddy water it will sort out the interference automatically. There has been times in swift water I would set my gain at a certain level to eliminate false readings floating under the boat. Check those settings.

Did it just start this? This a new graph? My buddy has a bird and it does pretty well. 👍
Sonar is just interpolation of reflected sound waves so you are possibly correct that the bridge pilings were affecting your return. There is also a pretty good chance that a soft bottom was messing with the echos and lots of little stuff (including minnows) were taking advantage of the shaded waters.
Thread starter #8
Liquid Nails, I don't really spend much time under bridges so it's hard to say if it's done that before but I'll start honing in on some settings and see if I can clear it up. I think it's in clear mode (as opposed to max mode) but maybe I need to turn the gain down.

Georgia Bob, one year when the water was down over 10 feet it was rip rap under the bridge as far down as I could see....but who knows, maybe it's mud on the very bottom. It was a little stained and there was some trash in the water but nothing too bad.

But am I right, if I'm over a large school the fish will be swimming up and down and it's show more as squiggly lines as opposed to long horizontal lines?


Senior Member
It only looked like that under the bridge. I was pretty close to the concrete pilings. Do you think that was what it was picking up?
I’m no world renowned expert but that would be my guess. If it only happened under a bridge close to a piling. I’ve seen all kinds of crazy images and this past weekend on the upper Tugaloo arm of Hartwell was a great example with moving muddy water with lots of large and small floating and slightly submerged debris.
Here is a pic where I just went under a bridge on Lanier a couple months ago. You can see the creek channel was the deepest spot and the ball of bait was about under the edge of the bridge.
Your sonar will pick up the bridge pilings. A lot of people don’t know that the sonar signals spread out on the way down. Like a cone with a curved bottom. You can actually take a ballon and tie a rock from a string attached to the ballon and put it 6 ft from your boat. And if the string is 6 ft long it will show the rock at 12 ft. I’ll post a link with information on this later.
The website is still up. It’s called Look under the depth finder interpretation forum. A guy called Woodlander posted some great information about depth finders and interpretation of what they show.


Senior Member
I think you are seeing the diminishing thermocline. The clutter from 15 ft down probably is the bottom water levels starting to turnover. Do you you really think it is that deep (check map contours)?. I suspect you might be seeing a false bottom at 33 ft due to overly high sensitivity.
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Thread starter #14
Procraft, thanks for the info. I'll check it out. BoosterC, I think the bottom depth is correct. If you look on a contour map the depth under Wahoo Bridge is give or take 30 feet. Maybe it's the thermocline AND the bridge pilings? :)
Procraft, thanks for the info. I'll check it out. BoosterC, I think the bottom depth is correct. If you look on a contour map the depth under Wahoo Bridge is give or take 30 feet. Maybe it's the thermocline AND the bridge pilings? :)
The bridge pilings are definitely showing up. I was on Wedowee under the 48 bridge and the pilings were showing on my depth finder when I was at least 75 feet from them. But the water there is right at 100 ft.
It is the thermocline. The lake bottom is yellow with the hardest return, the most dense deep water gives a stronger return in the red than the slight less dense thermocline in blue or the least dense upper level above it. As far as the difference under the bridge, when you get a thermocline in the summer, you will often get more “clutter” at night that during the day for some reason. You hear plankton cause it. I don’t really no why, but it happens. Likely similar case where you got more clutter under the shadows of the bridge that you did out in the sun. That isn’t from the pilings of the bridge. Adjust the sensitivity to make it go away.