Rickie's explanation also shows why you get "depth" readings way short of actual bottom depth, when you run across a seriously thick pile of brush. The sonar is picking up the topmost edge of the brush & "interpreting" it as the strongest signal, so it "thinks" it must be the bottom. It even, occasionally, happens when your transducer passes over a submerged log or big tree limb and all the sonar pings are returned from that "depth" ... even though there may be many more feet of water below the obstacle.

I'm referring, of course, to 2d & DI sonar pings ... as SI pings would likely not be affected in the same way ( but, because I don't have SI )

...cp