The water column is the distance from the boat to the bottom. The water column will get "wider" as you go over a drop off into deeper water.. You can also see the depth changes/shadows in the area that you are scanning.. In the pic below, look for the trees laying down "across" the old creek channel. The shadow (blocked signal of the "lay downs" lets you know that the trees are not resting completely on the bottom.. Look to the left side of the picture. That dark area is an old slough (deeper water) running alongside the creek. Look at the creek channel (water column) where my boat crossed the creek channel at the top and bottom in the center of the pic. You will see how the water column (depth) changes as the boat goes over the creek channel (gets wider)... Hope this helps..