Bottom Hardness

Bottom Hardness is a term used to describe the density (or composition) of the seabed (lake, riverbed etc). This is determined by interpreting the main reflected sonar signal from a depth sounder or fish finder.

A color echo sounder will use a different color for any one of up to 16 different signal strengths. This will make  Bottom returns show up as red and  weaker colors as green or blue. Monochrome sounders will use different levels of gray to show the different levels of signal strength. Bottom returns such as a hard bottom will show up as a very dark color and items such as debris in the water or concentrations of plankton will appear as a cloudy gray. Harder Bottoms (rock, reef) will show a Thicker Bottom Return whereas Softer(sand, mud) Bottoms will show as a Thinner Bottom Return.

A Sonar return from a sloping bottom can have the appearance of a softer bottom due to it also providing less return backscatter.