Depth sounder anomaly or fault ?

[nortonmotors]

My sounder has performed faultlessly for the last eight years, but caused me to wonder whether it is now faulty.
Beating up the Blackwater and crossing / re-crossing the deeper channel, I noted that the reading was reasonable at the margins but, when in the deeper part off Mersea (typically 12 -15 metres) the reading showed 30 - 50 metres for quite a while.
Was it the flood tide causing a layer effect / multiple reflections or a fault ?
Anyone else experience anything like this ?

 

[Tomkat17]

Had a similar problem after the harbour was dredged. It tooks a few months for the riverbed to settle to get an accurate sounding.

 

[sailorman]

what sounder is it

 

[sarabande]

sounds as if there might be double or triple bouncing of the signal, if the problem depth is a multiple of the known depth. Have you tried turning down the discrimination / sensitivity ?

 

[nortonmotors]

Its a Navico Corus depth / log instrument ; the log stopped working a year ago even though I've checked the paddle wheel is free to turn on several occasions.
On previous occasions I've been over that part of the river, the sounder has always agreed with charted depths which don't seem to have changed much in the last ten years or more.
If it happens again this weekend, I'll consult the book and see if there are any adjustments for sensitivity (when all else fails, RTFB).
Thanks to all for your comments.

 

[sailorman]

so it should "auto tune / compensate" then ie:- no gain knob