Shallow Water Settings for Lowrance HOOK?

Did some river fishing last Thursday and the river got shallow as I went upriver.  Spent quite a bit of time in 4 to 6 feet of water.

After a half hour the display changed.  The downscan display went from sepias to blues and the sonar page went from multi-color to gray with no detail(s).

The displays stayed the same when I went back downstream, in 30 feet of water.

At home, testing it to see if something was wrong, the colors and displays were normal again.

I'm guessing I spent way too much time in shallow water for my settings.

Does anyone routinely run a Lowrance is less than 10 feet of water?  If so, what settings are you using?