Humminbird LCR4 ID Operation Manual page 32

Brush or thick standing timber will appear as a thick
mass with holes or gaps as shown above. Notice
how the strong fish returns (red dots with black
behind) can be easily distinguished.
USING THE LCR
LEARNING TO READ THE DISPLAY
Thermocline is a temperature change at a
certain depth in the water. In some
conditions the sonar waves will actually
reflect or bounce off this temperature
change and therefore it will be displayed
on the LCR as shown above. To see the
thermocline, you will need to go to the
manual mode and increase the sensitivity
setting.
One of the best ways to learn to use your
LCR is to go over familiar locations. If you
know what's under the water and can see
it displayed on the LCR then you're on
your way toward gaining the experience
you need.